jl mcclellan: b-25 draft copyright 2005 p 1 of 27 · 2014. 1. 29. · packet, and messerschmitt...

JL McClellan: B-25 Draft Copyright 2005 p 1 of 27

Editors note:The Design Analysis article was originally

published in the March, 1945 issue, Volume 44,number 3, of Aviation magazine, published byMcGrawHill Publishing Company of New York, NY,USA.

This reconstruction is derived from microfilm. Thesource is University Microfilms International,Publication No. 364 (Aviation Week and SpaceTechnology), Reel No. 21 (January 1945 – December1945). The source was a tightly bound volume, so thatthere is some distortion of the images, especially nearthe binding. It has not been practical to remove orcompensate for all the distortions, so none of theillustrations in this reconstruction should be consideredreliable sources as to fine details of shape, proportion orspatial relationship. The distortions are, in general,small, and should not detract from a generalappreciation of arrangement and relationship.

The editor has attempted to represent the originallayout of the article, but there are some exceptions.Limitations in the compositing tools cause a differencein the text flow relative to the illustrations, compared tothe original, so that some changes have been made, tocompensate partially for that effect, and the tabular datahave been removed from the flow of text and broughttogether on a single page after the text, partly to makethem more accessible, and partly to sidestep problemswith page layout.

The Design Analysis article was one in a series ofdesign analyses published in Aviation during the waryears, between May 1943 and November 1945. Thesubjects were the Bell P39 Airacobra, Curtis C46Commando, Fleetwing BT12, Douglas A20 Havoc,Bristol Beaufighter (British), deHavilland Mosquito(British), North American P51 Mustang, Lockheed P38 Lightning, FockeWulf FW190 (captured German),Boeing B17 Flying Fortress, North American B25Mitchell (specifically, the B25H and B25J models),Mitsubishi “Zeke 32” Hamp (captured Japanese),Consolidated Vultee B24 Liberator, Fairchild C82Packet, and Messerschmitt Me262 (captured German),with one article dealing specifically with the Me262'sJumo 004 jet engine. Some of the analyses wereauthored by senior members of the design teams at theoriginal manufacturers, while others were written bystaff editors of Aviation magazine.

The original articles were copyright to theirrespective sources: the employers of the authors,following general practice of the time.

This reconstruction is compilation copyright JLMcClellan, 2005.

ROM THE OUTSET of thewar, the North American B25 Mitchell medium

bomber series has earned anenviable reputation in the hands ofUnited Nations pilots in all theatersof combat. Designed in 1939 andfirst flown in 1940, this plane hasbeen developed through a series ofchanges brought along by practicalneeds.

F

Latest models of the Mitchell toenter service are the B25H,outstanding among productionplanes for its extremely heavyarmament and the B25J, latestimprovement on the originalmedium bomber design.

These are pointed examples ofNorth American's design policy ofease and economy of fabrication. Inthis instance, the Mitchell wasmanufactured on ___ 48 majorassemblies, any one of which ,theoretically, could be modifiedwithout necessarily affecting theother 47. The B25 has undergone

nearly a dozen such changeswithout altering the basic design.

In both models, firepower hasbeen increased by addition of a tailturret, waist guns, moving th upperturret forward to improve field offire, and two fixed forwardfiringpackage guns on each side of pilot'scompartment.

Nose of the B25H is fitted withfour machine guns and a 75 mm.cannons; the B25J with twomachine guns – one flexible andone fixed – protruding from thebombardier's enclosure.

Tactical purpose of the B25H isprimarily for low level attack and

the destruction of land or navalmateriel targets, in support ofground, air, and naval forces. As adual purpose plane, it can knock outsurface forces protecting the target,then proceed to bomb the targetfrom low altitudes. The B25J isprimarily a medium bomber,nevertheless, it is capable of dealingwith surface forces.

In spite of their versatility,current B25's virtually remainunchanged from the original. Theyare semimonocoque midwingmonoplanes powered with twoWright 1700hp. 14cyl. engines.All fixed surfaces are metal


DESIGN ANALYSIS OF

The North American B-25 Mitchell

By C.J.Hansen, Chief Project Engineer, North American Aviation, Inc.This distinctive gun-bristling medium bomber ― first warbird todrop “eggs on Japan ― has again and again played a strikinglyeffective role in far-flung war performances. Here's a meatypresentation of basic B-25 makeup features and comparativedetails of models H and J . . . The 11th in AVIATION's peerlessdesign analysis series.


Three-view aspect of North American B-25J Mitchell medium bomber. B-25H, withall metal nose and 75 mm. cannon, has dimensions identical to those shown aboveexcept that distance between centerlines of nose wheel and main landing wheels is2 in. greater and overall length is 2 ft.-2 in. less.

covered. Flight control surfaces,exclusive of wing flaps, are heavilydopedfabric covered members.Countersunk rivets are used on theforward one third of fuselage skinand fixed surfaces to reduce drag,and brazier head rivets on the afttwothirds.

Normal entrances and exitsconsist of two hatches in theplane's belly, one under the upperturret compartment, the other aft ofthe waist gunner's position.Automatically retractingstepladders are provided.

Escape hatches are for use in

crash landings when main entrancehatches are blocked, or underemergency conditions while theairplane is in flight.

Crew for the B25H consists of apilot, upper turret gunner,cannoneernavigator, waist gunnerand tail gunner. Pilot, copilot


Exploded view showing details of B-25H and B-25J. Legend: (1) 75 mm.cannon installation, (2) four fixed .50 cal. nose guns (B-25H), (3) noseassembly hood, (4) bulkhead, (5) .50 cal. blister guns, (6) pilot'senclosure, (7) fuselage front, side frame, (8) Bendix upper turret, (9)Nose assembly, inboard center section, (10) fuselage front, top frame,(11) bomb bay door, (12) fuselage top frame, (13) bomb bay passagewayfloor, (14) rear fuselage side frame, (15) life raft, (16) rear fuselage, topframe, (17) horizontal stabilizer upper fairing, (18) tail gunner's canopy,(19) elevator, (20) Bell tail turret, (21) tail gunner's compartment, (22)rudder, (23) vertical stabilizer, (24) horizontal stabilizer, (25) rearfuselage, bottom frame, (26) rear entrance hatch, (27) inboard flap, (28)nacelle, (29) outboard flap, (30) .50 cal. waist gun, (31) aileron, (32) wingcenter section, (33) nacelle upper fairing, (34) main landing gear doors,(35) outer wing panel, (36) wing tip, (37) main landing gear, (38) noseassembly, outboard center section, (39) cowling, (40) powerplant, (41)pilot's floor, (42) front entrance hatch, (43) fuselage front, bottom frame,(44) fuselage beam, (45) lower nose assembly (B-25H), (46) noselanding gear door, and (47) nose landing gear. Nose section of B-25Jshown at lower left has fixed and flexible .50 cal. guns.

navigator, bombardier, upper turretgunner, waist gunner, and tailgunner man the B25J.

Both bombers now carryconsiderably more armor plate toafford maximum protection for thecrew and vital equipment. Providedalso, is a rubber lifeboat, emergencyequipment, pyrotechnics, and theusual oxygen equipment.

Center Section Structure

Backbone of the B25 is the bombbay section, built integrally with thewing center section. To resist amajor portion of fuselage bendingloads carried by this section, 24STHshaped extruded lower longeronsand Zshaped upper longerons areemployed. Fore and aft fuselageframes, to which longerons attach,are composed of 24ST webs rivetedto rightangle alloy extrusions.These frames are attached to frontand rear wing spars and serve to

introduce flight loads into thefuselage.

Intermediate similarlyconstructed frames serve asattaching members for the bombrack structure which is stressed tocarry part of the fuselage loads.Top fuselage frames in this sectionsupport bomb hoisting loads.

Sides of the bomb baycompartment are additionallysupported by intermediate frames,plate ribs, rolled Alclad stringers,and channels. Bomb rack railswhich support bomb loads arebolted to bomb rack attachmentframes. Roof of this compartmentalso serves as the floor of acrawlway which permits crewtravel fore and aft during flight.This floor is .032 24ST Alcladstiffened by hatshaped rolledangles and extrusions. Transverseshear loads are borne by the flooralong with the top fuselage skin.At this section the fuselage is

skinned with lapjointed 24STAlclad varying from .032 on top to .051 on the sides.

Bomb bay doors have inner andouter skins. Inner skin is a singlepanel lightened by circular cutouts ,attached to a framework of stringersand frames. Outer skin is riveted toand supported by the sameframework. Doors are hinged to theadjoining structure at three points.

Wing Center Section

The wing center section isattached to and supported by theintermediate fuselage section orbomb bay compartment. Two mainspars, front and rear, serve to resistspanwise bending loads and extend157 in. on either side of fuselagecenterline.

Spars consist of 24ST aluminumalloy webs, .081, .064, and.051, towhich extruded capstrips are riveted.A firewall of .019 stainless steel acts


B-25J front fuselage frame assembly: (1 and 2) Bombardier's enclosure and escape hatch,respectively, (3 and 4) pilot's compartment enclosure and escape hatch, respectively, and (5)fuselage front lower assembly.

as a doubler and extends along theforward side of the front spar behindthe engines.

Landing gear beams betweenfront and rear spars begin t a point97 in. from the fuselage centerline,and consist of .051 24ST Alcladwebs and vertical 24ST extrudedstiffeners. Attached to the lowerpart of landing gear beams are gearlock fittings and vertical bracefittings. Vertical load at thesefittings is applied to the web bymeans of adjacent vertical stiffeners.Purpose of the landing gear beam isto resist vertical loads when landingand while the plane is supported onthe ground by means of the landinggear.

An intermediate beam, known asthe bas web, is located betweenfront and rear spars and extendsoutward from the fuselage for adistance of 97 in. on each side.This member separates the twomain fuel compartments and carriessome of the flight loads. It consistsof an .081 24ST Alclad web and24ST hatshaped stiffeners rivetedat 10 in. intervals.

Six center section ribs distributea major portion of landing gearfitting, landing gear beam, andengine mount fitting loads. Wingjoint platetype ribs at each end ofthe center section serve toredistribute the normal shear andtorsional moment from outer panel

to center section. They are formedprincipally of .064 24ST Alclad, andare supported vertically bystiffeners. Similar ribs placed 97and 133 in. from either side offuselage centerline are made,respectively,of .064 and .091 24STAlclad and aid in distributinglanding gear and engine mountfitting loads.

Upper center section surface isskinned with 24ST Alclad from .032to .081 thick, riveted to ribs andtransverse stiffeners, buttjointedspanwise and lapjointed chordwise.Lower center section surface is fittedwith .051 and .064 24ST panels,removable to provide access to fuelcells located in the wing. Panels are


B-25H front fuselage frame assembly: (1) Hood,(2) cowling, (3 and 4) pilot's enclosure andescape hatch, respectively, (5) fuselage top,turret gunner's compartment, (6, 7) fuselagefront side assemblies, (8) armor plate floor, (9)

deflector plate, (10) pilot's floor, (11) turretstructure support, (12) fuselage front bottomassembly, (13) front entrance hatch cover andladder assembly, (14) nose wheel door, and(15) lower nose assembly.

stiffened chordwise by hatsectionedformers and by spanwise stringers.Fuel tank securing fittings areattached to inner surface of thepanels.

Center section leading edge isconventionally constructed of rolled24ST Alclad skin stiffened bystringers and supported by formednose ribs. Inboard flaps constitutethe center section trailing edge.

Fuselage Forward Section

Forward fuselage sectionextends aft from the nose to thebulkhead at front end of the bombbay. This section consists of thehinged hood and fixed lower“front” section on the B25H, orbombardier's compartment on theB25J, and pilot and upper turretcompartments on both models.Main bulkhead for support of nose

gear fittings and pilot's compartmentarmor plate fittings, is located at rearof front section.

Pilot's cockpit enclosure is madeof Lucite or Plexiglas, and glasssheet, set in metal frames and slidingpanels. Floor section is an integralstructural feature of the fuselage.The 75 mm. cannon on the B25H isin the crawlway used by thebombardier on the B25J. Thecannon housing is on the left side of


Fuselage intermediate frame assembly: (1) Compositestructure, (2) top frame, (3) crawlway floor assembly,(4) center section covered assembly, (5) bomb bayfuselage side assembly, (6) bomb sling door, (7) fuelcross-feed bracket, (8) hydraulic reservoir support, (9 to12) wing nose assemblies, (13) nacelle, aft, (14)nacelle, forward, (15 to 20) main landing gear doors,and (21, 22) bomb bay doors.

the pilot's compartment under thecockpit floor, and serves,structurally, as a torquebox.

Cannoneer's compartment on theB25H is known as the upper turretcompartment on the B25J, and islocated behind the pilot's enclosure.It includes the front entrance hatch,upper turret guns, and on the B25His used as the loading station for thecannon.

Four longerons are the largestsingle loadcarrying fuselagemembers, and bear axial tension andcompression forces due to action ofbending. Shape and size of eachdepends on the load carried, andexcept for the extruded longerons inthe bomb bay assembly, they consistof channels formed from 24SOAlclad sheet later heat treated to24ST.

Two lower longerons extend thefull length of the fuselage. They arecontinuous within individualfuselage sections later boltedtogether to form a running fuselagestructure. In the fixed portion of thefront forward section, the leftlongeron (on B25H and J) is a Zshaped member formed of .051Alclad (later heat treated). Becauseof higher load factors, thickness ofthe corresponding right lowerlongeron is increased to .091.

From the front forward sectionextending back through pilot'scompartment to front bulkhead ofthe bomb bay section, lowerlongerons are downwardflanged .064 24ST Alclad channel members,reinforced along the skin side flangeby extruded angle. In approximatelythe middle of pilot's compartment an.081 rear section is spliced in witha .064 doubler of the same material.

True upper longerons in theforward section are located onlybetween the bomb bay and pilot'scompartment rear bulkhead and are .091 24SO formed channels heattreated to 24ST. They are otherwisereplaced by members known asbombardier's rails and pilot's rails,and serve as upper extensions of thelongerons. “Breaking” of the upperlongerons and continuation of thestructural frame on a lower plane ismade necessary by the cutout for theglass enclosure in pilot'scompartment.

Each pilot's rail runslongitudinally along the side of thefuselage, just below pilot's side

window. Rails are attached to thefuselage finishing angle at the endof the gun turret compartment,extend forward, and are connectedtot he front bulkhead of pilot'scompartment. Each is adownwardflanged Lshapechannel member of .091 24SOAlclad sheet (later heat treated) towhich an extruded angle is riveted.Forward and aft of a bulkheadlocated approximately at the firstframe of pilot's enclosure, loadsdiminish in intensity and thedownward flange of the extrudedangle is machine tapered.

Pilot's compartment glassenclosure frame is secured by wingchannel nuts riveted to the innerflange of pilot's rail.

Second forward extension ofupper longerons is provided by

bombardier's rails which extend oneach side of the fuselage from abulkhead in line with first pilot'senclosure frame to the front of theforward front nose section on the B25H.

On the B25J, these railsconstitute the base of bombardier'stransparent enclosure, serve as themain structural members of the nosesection,a nd are on a lower planethan pilot's rails. From the frontbulkhead of pilot's compartmentforward, they are downwardflanged.051 24SO Alclad channel members(heat treated to 24ST after forming)and are .064 thick, rearward. Exceptfor armor plate, fuselage is skinnedwith 24ST Alclad ranging from .051to .025.

Rear fuselage section includes theportion aft of the bomb bay


Fuselage rear frame assembly: (1) Side frame assembly, (2) life raft door, (3)top assembly, (4 and 5) tail gunner's compartment and canopy, respectively,(6) bottom assembly, (7) waist gun window, and (8) rear exit hatch.


Details of outer wing panel structure: (1) Trailing edge rib, (2) aileron falsespar, (3) intermediate rib, (4) main spar, (5) nose rib, (6) former, (7) landinglight, (8) oil temperature regulator location, (9) bolting angle, (10 to 13) boltingangle supports and attaching joints, and (14) flap false spar. Detail (A) showsfeatures of flap hinge bracket, and details (B) and (C ) represent aileronoutboard and inboard hinge brackets.

Structural features of wing tip: (1) Forward bow, (2) front former, (3 and 4)front and rear intermediate formers, respectively, (5) rear former, (6) rear bow,and (7) rib.

assembled about a main spar locatedat the 33 percent chord line.Secondary false spars extend thelength of the panel along its trailingedge. Remaining structure consistsof ribs pressed from 24SO and 24STAlclad sheet, extruded spanwisestringers and 24ST Alclad skin.Access and inspection doors areprovided along the entire wing, mostbeing on the lower skin.

Outer main wing spar consists ofinboard and outboard sections of24ST sheet spliced with a .064Alclad doubler plate. Inboard sparsection is .081 material formed withupper and lower Ushaped flangesserving as spar caps. The web isstiffened by vertical rib attachingrightangle extrusions andindependent stiffener angles.

Outboard of the wing centersection attaching plates, the web iscut away to permit installation oftwo oil cooling ducts. Each cutoutis reinforced by a Tsection extrudedring riveted around the cutout.Inboard spar section is additionallyreinforced by an .081 24ST Alcladdoubler extending between the ribson either side of he cutout. Theoutboard flanged web is .064 Alcladwith upper and lower edges formedto provide spar caps. Eight flangedlightening holes are provided in thespar tip.

The outer wing flap false spar is .040 24ST Alclad with formed rightangle flanges for spar caps.Between the endplate and the thirdrib, the spar of a single sheet of .04024STsection riveted to upper andlower spar caps. Inboard websection has several cutouts toaccommodate equipment.

Ailerons are supported by a falsespar of a single sheet of .040 24STAlclad. Formed rightangle flangeson the upper and lower edgesprovide spar caps. The web isstiffened by several rib attachingangles of extruded alloy arrangedvertically along the solid web.

Majority of the wing ribs arepressed from 24ST or 24 SO Alcladvarying from .025 to .040. All ribsforward of the main spar are


Structural features of wing tip: (1)Forward bow, (2) front former, (3 and4) front and rear intermediateformers, respectively, (5) rearformer, (6) rear bow, and (7) rib.

Structural details of horizontal stabilizer:(1) Rib, (2) front spar, (3) front sparfitting, (4) to 12) brackets, (13) rear spar,(14) to 19) elevator hinge brackets, (20,21) elevator horns, and (22) elevatorconnecting tube.

provided with flanges riveted to theskin. Most intermediate ribsbetween main spar and false sparsare formed so they do not contactthe skin but are attached to stringersby means of clips. All ribs areprovided with conventionallightening holes and stiffeningbeads.

Wingtips are constructed of threeribs and several formers, all pressformed from Alclad sheet stock.Beaded lightening holes, stringercutouts, and flanges are provided ineach member. Contour skin isformed from 24SO and tips arecovered with 24ST Alclad and

attached to the wing with screws.Horizontal stabilizer is a full

cantilever stressed skin structure ofpressed ribs, spanwise stiffeners,and two spars. Front stabilizer sparis a web of .051 24ST Alcladspliced to a heavier web of .064material where it covers thefuselage. Beaded lightening holesand angle vertical members stiffenthe web. Top and bottom capstripsare extruded angles riveted intoplace. Rear spar is of substantiallythe same construction except thatthe web is .051 throughout.

All ribs are made of .032 24STAlclad with flanges acting as caps

and stiffening beads pressed informing. Lightening holes andstringer cutouts are also provided.Stringers are extruded 24ST bulbangles and formed Jsections.Stabilizer skin is .025 and .032 24STAlclad riveted to the framework.

Vertical stabilizers are similar instructure and are attached to thehorizontal stabilizer with standardAN bolts at the junction point offront and rear spars.

Movable Surfaces

Wing flaps of the B25H and J arethe trailing edge slotted type,consisting of two sections on eachside of the fuselage. Inboard flapsextend from the nacelle tail cone tothe side of the fuselage, andoutboard flaps extend from theaileron to the nacelle cone. When inneutral position, flaps are sealed bya nonmetallic strip attached to theupper wing surface, and large slotopenings on the lower surface of thewing are closed by small fairingdoors.

When flaps are moved to a downposition, fairing doors swing upwardpermitting flow of air up betweenthe fixed wing and the lowered flap.Rubbing strips of .010 halfhardsteel sheet are provided along theflap leading edge at two places.Actuating torque of each flap istaken by a torque tube whichextends into its interior. Flaps aresupported by, but not hinged to thetorque tube.

Flaps have powerpressed ribsand spanwise stiffeners. Ribs areprovided, with stringer cutouts andlightening holes. Trailing edge isformed of .032 24ST Alclad.

Sealed type ailerons have a frontspar and pressed ribs. Conventionalweights are attached to the leadingedge to obtain static and dynamicbalance.

Aileron main spar is 137 in. and isformed from .032 24ST Alclad sheetinto a Ushaped channel, joggled onthe outside upper and lower capedges to provide for attachment offabric replacement strips. Ribattaching angles are placedvertically on the spar web.Lightening holes are spaced alongits entire length. A light false spar isprovided as a trim tab attachingmember. Ribs are formed from .032 24ST Alclad and include


Structural details of vertical stabilizer: (1) Tipassembly, (2) front spar, (3) lower tip control, (4) rearspar, (5) rudder control sheave support, and (6, 7)rudder hinge brackets.

formed top and bottom frames andbeaded lightening holes.

A trim tab, mounted in the trailingedge of each aileron, consists of asmall metal airfoil, of rectangularplan, having a Ushaped spar,triangular shaped ribs, and an Alcladcovering. Hinged at three points tothe aileron false spar, the tabs serveas control boosters in addition totrim units and are not balanced.

Both elevator and rudder spars areformed of .040 24ST Alclad with7/8 in. flanges along top and bottomsurfaces. Flanged holes lighten thestructure and stiffen the spar web.Rudder and elevator false spars areUshaped, formed from .032 24STAlclad. Trailing edge ribs attach tothese members, and brackets areused to attach trim tabs at threepoints along the flat surface of thespar. Nose skins for both membersare rolled from .025 24ST Alcladand are riveted to main spar flangesand rib caps. Rudder nose skins areprovided with flanged lighteningholes. Rudder and elevator trailingedges are Ushaped members of .025 Alclad riveted to the rib tips.Trim tabs are constructed of sheetmetal in the conventional manner.

Fabric covering of the elevatorsand rudders is grade A mercerizedcotton, doped and finished.Covering is attached to dimpledholes in the trailing edge ribcapstrips[sic] by countersunk sheetmetal screws inserted throughdimpled washers. Dopedreinforcing tape is placed along theribs before the screws are inserted.


Structural details of outboardand inboard wing flaps. DetailA shows features of fairingdoor assembly.

Flap installation, showing details ofemergency lowering mechanism.

After insertion of the screws andwashers, a strip of finishing tape isplaced over them to provide asmooth surface.

Nacelles and Engine Mount

Engine nacelles are broken downinto three main divisions – enginecowling section, front section, andrear section. Engine cowling sectionconsists of a nose ring, cowlformers, and cowl panels. Materialis aluminum alloy and corrosionresistant steel. The entire structureis supported by the front sectionframes bolted to the bottom of thewing center section.

Nacelle front and rear longeronsare rolled Zsection members .091and .064 24SO Alclad, respectively,heat treated to ST condition afterforming. Frames are typical pressedmembers of 24ST Alclad of varyingthickness and have rightangleflanges and stiffening beads foradded strength.

The firewall web is made ofstainless steel sheet, spotwelded to aseries of stiffening members ofvarying size and cross section.Holes are provided to accommodateelectrical and hydraulic lines.

The engine mount is X4130chrome molybdenum steel tubing,arc welded int a single unit, boltedto the wing structure at four points..The engine is attached to themounting ring by cushioneddynamic fittings. Tube diametersrange from 11/4 to 2 in. O. D. withwall thickness varying from .049 to .094.

The B25 was the first productionmedium bomber type to incorporatethe now widely used tricycle landinggear.

Main landing gear is of the singleleg, halffork, fullcantilever designand is fully retractable into theengine nacelles. Each shock strutconsists of a cylinder and piston,shock being absorbed by air and oil.Piston and cylinder areinterconnected by forged chromemolybdenum steel torsion linkswhich transmit torsional momentsfrom piston to cylinder. Torsionalloads applied during landing aretransmitted through the shock strutto the side thrust brace, twotrunnions, the downlock and themain supports.

A 47in. tire is mounted on each


Aileron frame assembly. Detail A showsinboard hinge bracket with heavy-dutybearing.

Elevator frame assembly: (1) Rib, and (2 and3) short and long counterbalance weights,respectively.

of two cast aluminum alloy dropcentertype wheels carried onhardened steel axles over which arepinned chrome molybdenum steelforks. All chrome molybdenumsteel landing gear parts are heattreated to a tensile strength of160,000 – 180,000 lb / sq. in. Eachwheel of main landing gear isequipped with hydraulicallyoperated, multipledisk metaltometal clutch type brakes.

Nose landing gear consists of anairoil shock strut and 30in. wheel.Like the main landing gear, the nosegear is fully retractable. In retractedposition it rests in the fuselageunderneath and to the right of the

pilot. All torsion and bendingloads are transmitted and reacted inthe same manner as they are in themain gear. The nose wheel has asteering range of 60 deg. left orright, allowing a turn with eithermain gear as a pivot point. Ahydraulic shimmy damper ismounted on the strut which alsoincorporates a selfcentering deviceto center the wheel when theweight of the plane is removedfrom the nose gear.

Hydraulic System

The hydraulic system – singlepressure type – operates thetricycle landing gear, wing flaps,cowl flaps, bomb bay doors, andbrakes. Cowl flaps have separatecontrol handles for left and rightengines, and operation of either thecowl or wing flap may be stoppedat any desired position. Amanually operated emergencyhydraulic system permits actuationof the different subsystems shouldboth enginedriven pumps fail, orwhen the airplane is on the groundwith engines not operating.

The Pesco 349P hydraulic pumpsare enginedriven, twogear, positivedisplacement types with a pressurelimit of 1,500 lb./sq. in. forcontinuous operation, will operate ineither direction, and are completelylubricated by the fluid passingthrough. They are located on theengine accessory sections and eitheris capable of providing sufficientpressure for the system.

Fluid from the reservoir is forcedfrom the pumps through lines routedback of the forward wall of thebomb bay where the lines from bothpumps join. Check valves areprovided so that failure of one pumpwill not render ineffective thepressure produced by the operatingpump. The pressure line continueson from a Tfitting through a checkvalve and then through all thesystem lines leading to the selectiveoperating systems.

Fuel System

An independent fuel system isprovided for each engine. Mainsource of fuel supply is from fourselfsealing tanks, two located in


Rudder frame assembly: (1) Typicalnose rig, (2) torque tube, (3) horncutout fabric support, (4) typicaltrailing rib, (5) tab bracket, (6) falsespar, and (7) false spar reinforcingtubes.

Nose ring and removable cowling: (1) Enginenose ring, (2) link, cowling to engine, (3) cowlformer, (4) engine flap ring, and (5)removable cowl panel.

each wing center section betweenthe fuselage and the engine nacelle.Front and rear tanks on each side areconnected by a line from the reartank to an adapter to which a boosterpump is attached. A check valvepermits fuel to flow from the rear tothe front tank and then to the engine,but prevents fuel from returning tothe rear tank.

Booster pumps are operated byswitches on pilot's control pedestalswitch panel. An auxiliary fuelsupply is also obtained from sixsmall selfsealing tanks, three ineach wing center section outboard of

the main tanks. Front and outboardtanks in each group are connectedwith the rear auxiliary tank by aline from each tank aft through thenacelle to an electrically operatedtransfer pump. The transfer pumpdraws fuel simultaneously from thethree auxiliary tanks, pumps itthrough a line to the front maintanks.

Each engine is fitted with typeG9 rotary vane positivedisplacement type fuel pumplocated on the righthad aft end ofthe engine.

Oil System

Each engine has an independentoil system, identical except forminor variations in oil line routings.A selfsealing tank is installed in thewing section aft of the enginefirewall. Oil flows from the tank tothe enginedriven oil pump, to theengine under pressure of 8090lb./sq. in.

System oil is utilized forfeathering the propellers and is takenfrom the circulating oil by a pump,mounted on the front of the engine.Temperature of the oil returning


Schematic diagram of hydraulic system.

from the engine is regulated by twothermostatically controlled type C8oil temperature regulators mountedin the center portion of ducts whichopen in the leading edge of the wingand have outlets through the uppersurface of the wing just forward ofthe wing flaps.

Surface Control SystemA duplicate cable control system

is installed to actuate rudders,elevators, and ailerons. Each systemis so designed that loss, throughgunfire, of any one cable will not

seriously cripple the plane. Cablesare colorbanded to facilitateassembly, repair, and inspection ofthe control system.

Control forces originated by thepilot are applied to the controlcolumn, located on the left side ofthe compartment, connected to asteel torque tube extending acrossthe fuselage and having takeoffhorns at each end. Elevator cablesextend aft along each side of thefuselage from control columnhorns to bellcranks in the aftfuselage. Adjustable pushpull

rods connect each bellcrank to therespective elevator horns.

Elevators are joined by a torquetube connected to each elevatorhorn, and may be raised 25 deg. orlowered 10 deg with respect to thehorizontal stabilizer. A bungee isincorporated in the elevator controlsystem to reduce forces on thecontrol column.

Ailerons are controlled byclockwise and counterclockwisemovement of pilot's control wheel.Cables are led out of the controlcolumn torque tube, then aft to the


Layout showing details of fuel system.

aileron sector on the rear wing spar.Maximum upward aileronmovement is 28 deg., downward is14 deg.

Rudders are actuated by hangingtype pedals. Control cables extendaft along each side of the fuselagefrom the lower outboard ends of therudder pedal assembly to thehorizontal stabilizer, then outboardon each side of the stabilizer torudder sheaves at the outboard endsof the stabilizer. Rudders can bemoved 20 deg. right or left withrespect to vertical stabilizers. Lossof cables on one side of the airplanewill not affect rudder control on theopposite side.

Elevator trim tabs are operated by

control wheel on the left side ofpilot's control pedestal. Ailerontrim tabs are actuated by theforward control knob on the floorof pilot's compartment, and the aftcontrol knob on pilot's flooroperates the rudder tabs.

Power Plant and Propeller

B25H's and J's are powered bytwo Wright 14cyl. twinrow R2600 engines, each rated at 1,700hp. for takeoff. The exhaust

system uses Claytontype flamedamping individual stacks whichprotrude through openings in thecowling. The Jack & Heinz JH5Fstarter may be used as aconventional inertia starter todirectly crank a warm enginewithout energizing, or to directcrank after energizing, if desired.

Ignition is provided by twoScintilla SF14L3 magnetos with14lobe breaker cams turning at halfcrankshaftspeed. Both magnetosare timed to No. 1 cylinder, and


Pilot's lower control pedestal: (1) Aileron and rudder tab controls, (2) hydrauliccontrol cap assembly, (3) rudder tab indicator plate, (4) hydraulic hand pump,(5) pneumatic emergency brake valve, and (6) pneumatic emergency brakehandle. Complete pedestal is seen at lower right.

each is cooled by air from blasttubes running forward to the front ofthe engine.

Each engine is equipped withModel 1685HA Holley carburetorto which flow of air is directed by ascoop mounted to the intake by aflexible joint. Main body of the airscoop is a casting containing a coldair door interconnected by linkageand controlled from pilot's controlpedestal. A removable coverpermits installation of either filter orbaffle. When the cockpit control isin the Normal position, cold air isadmitted to the carburetor, and whencontrol is in Icing, warm, unrammedair enters from the enginecompartment.

Power plants are equipped withthreebladed Hamilton StandardHydromatic, fullfeathering,constantspeed propellers, diameter12 ft. 7 in. Blade settings rangefrom low pitch of 22 deg. to high of


Pilot's upper control pedestal: (1and 2) Main control quadrant androd assembly, respectively, (3)auxiliary control quadrant, (4)throttle and mixture rod, (5 and 6)right and left carburetor heatcontrol rods, respectively, (7)control pulley base. Pedestalassembly is seen at lower left.

Nacelle engine controls: (1) Pulley bracket, (2) carburetor aircontrol rod, (3) propeller control cable, (4) mixture control rod, (5)mixture control level, (6) throttle control lever, (7) propellergovernor control bracket, (8) propeller governor, (9) superchargerair control rod, and (10) engine mount.

90 deg.

Electrical System

B25 H and J electrical systemsare 24 v. d.c. singlewire type withthe airplane structure serving as acommon ground except where dualwiring is required to preventcompass deflection. Nearly all ofthe wiring is open type supported byclips and protected, wherenecessary, by insulating tubing,tape, or cord. Conduits enclosewiring in engine nacelles andwherever additional mechanicalsupport or electrostatic shielding isnecessary.

A 24 v. 34amp. hr. battery islocated in each of the enginenacelles, aft of the firewall. Eitherbattery will operate the electricalsystem, including starters.

Two enginedriven 200amp. 30v. generators, one mounted on thesupercharger housing of eachengine, power the electricalsystem. Each generator is cooledby a blast tube leading to thegenerator from just aft of thepropeller disk.

Radio Equipment

Radio system consists of thecommand set used for planeto

plane communication; radiocompass receiver; multiplaceinterphone system; and emergencytransmitter for use in the life raftshould the crew be forced down atsea. The B25J has a liaison set, forlong distance planetoplaneoperation, as additional equipment.

The command set includes a rackwith two transmitters, rack withthree receivers, combineddynamotor power supply andmodulator, antenna relay, andnecessary command radio wiring.With exception of control boxes, thecommand set is located in the upperleft forward portion of cannoneer'scompartment on the B25H, and in


Shown here is B-25H installation of four fixed nose guns.Legend: (1) .50 cal. machine gun, (2) gun cover, (3) guncharger pulley bracket, (4) chute, (5) post assembly, (6)solenoid, (7) ammunition box, (8) roller, (9) ammunition feedchute, and (10) trigger switch. Insets show ring and bead sightcomponents.

upper turret gunner's compartmenton model J.

Radio compass consists of areceiver, remote control box,azimuth indicator, rotatable loop,and terminal junction box. Thereceiver unit is a 15tubesuperheterodyne with a widefrequency range and is located onthe forward portion of the cockpitfloor, on the right side of the plane.

Emergency transmitter, consistingof a portable unit with selfcontained

generator, 300ft. antenna, kite,balloon, signal lamp, hydrogengenerator, and parachute attachedto two canvas bags in which theequipment is stowed, is used bypersonnel forced down in thewater, and is pretuned to theinternational distress frequency of500 kc.

Interphone equipment includesan amplifier, one jack box for eachof the seven stations, and throatmicrophone and headset for each

crew member.Liaison set consists of a receiver

and transmitter.

Gunnery Equipment

The B25H has fourteen .50 cal.machine guns and one 75 mm.cannon, and the B25J mountstwelve .50 cal. guns.

The cannon assembly in the B25H, consisting of type T13weapon mounted on type T13E2


75 mm. cannon installation (B-25H): (1) Sight, (2) trigger switch,(3) firing mechanism actuator, (4)ammunition stowage rack, and (5)cannon mount.

recoil mount, is situated in thetunnel beneath the left side of thepilot's compartment. Cannonmuzzle projects forward through ablast tube in the lower nose sectionand the breech extends aft to the leftforward side of cannoneer'scompartment. Cannoneer loads thegun and pilot fires it.

The allmetal nose forms acompartment for the four nose gunsand ammunition boxes. Upperportion of the nose is hinged toprovide access to the guns andammunition belts. Guns aremounted side by side, are chargedby cannoneer and fired by pilot.

Two guns are installed on eachside of the fuselage just outsidecannoneer's compartment. Attached

to metal brackets, they are enclosedby metal blisters fastened tofuselage. Ammunition boxes aresupported on a shelf structure ateeach side of cannoneer'scompartment. Blister guns are alsofired by pilot.

The upper turret is installed on asupport pedestal in the aft portionof cannoneer's compartment. Thefield of fire of the two .50 cal. gunsin azimuth and elevation isautomatically controlled by camsand switches in series, allowinggunner to freely follow a targetwithout bullets striking any part ofthe plane.

Waist guns are flexible, mountedon a trunnion yoke, and projectthrough a canvas boot in a

transparent glass blister, one on eachside of fuselage aft of the bomb bay.Each gun is provided with bungeecables to aid gunner in maneuveringduring firing periods. Ammunitionis stored on a shelf aft of the gunsand is led to them through fixedfeed and flexible chutes.

Tail installation consists of theBell Type M7 turret with a twingun adapter mounting two TypeM2 .50 cal. machine guns. Feedchutes are equipped with boostermotors.

In the B25J, the four nose gunsand cannon are replaced by a fixedforwardfiring .50 cal. gun and oneflexible .50 cal. gun. The flexiblegun, operated by th bombardier, isplaced in a ballandsocket mount


Fixed blister-gun installations (left and right side): (1, 2) Cartridge feed chuteretaining strap, (3, 4) feed chute, (5) .50 cal. machine gun, (6) link ejectorchute, (7, 8 and 9) gun fairing assembly, (10) solenoid, and (11) charger.

installed in the foremost point of thenose directly above the bombsighting window. The fixed nosegun is mounted on the right side ofbombardier's compartment, it'sbarrel projecting through the nose,and is fired by the pilot. Armamentof the B25 is otherwise identical

with that of the B25H model.

Bombing Equipment

The bomb bay is equipped withfixed ladder type racks designed toaccommodate 100 to 1,600lb.bombs. A special rack may be

installed to carry one 2,000lb.bomb, or another special rackinstalled to accommodate a torpedo.Controls enable pilot to releasebombs from the racks eitherelectrically or mechanically. Amanhole is provided for ease instringing bomb hoisting cables and


Details of flexible waist-gun installations: (1) Bungee cable, (2)ammunition chute, (3) rollers, (4) balance assembly, and (5)ammunition boxes.

permits inspection of the bomb bayduring flight.

Oxygen Equipment

Two portable lowpressureoxygen units are stowed at the rearentrance of the bomb bay crawlway.

When in use, one is mountedbehind pilot's seat and the other isattached to the forward corner ofcannoneer's compartment ceiling.

Type AN R5 demand regulatoris mounted on the side of eachoxygen cylinder and automaticallycontrols flow and dilution of

oxygen. As the user inhales, adiaphragm is inflated, opening avalve permitting oxygen to flowthrough the regulator. Oxygen thenmixes with free air in an amountgoverned by an aneroid valve whichcontrols an air port and an oxygenport. At sea level the air port isopen and the oxygen port is closed.As the altitude increases, the aneroidexpands, closing the air port untilfinally, at about 30,000 feet, the airport is completely closed and theregulator is delivering pure oxygen.

Instrument andPhotographic Equipment

Instruments on the B25 aredivided into four generalclassifications: Vacuum system,aairspeed system, engine system andmiscellaneous. Normal complementof instruments applicable to a twinengine bombardment plane isutilized and offers no unusualinstallations.

A type K24 camera is locatedjust aft of the bomb bay in thefuselage rear section. Photographsare taken through a window in the


Upper gun turret installation: (1) Master upper turret, and (2, 3 and 4) azimuthresistor box, cable to motor amplidyne, and cable to brush box, respectively.

Tail turret installation: (1) Ammunition box,(2) feed chute, (3) ammunition booster, (4)hydraulic remote control assembly, (5) tailturret curtain, (6) .50 cal. machine gun and(7) ejected ammunition can deflector.

fuselage floor, through a range of 50deg. fore and aft of vertical.

Cold Weather Provisions

Three interchangeable StewartWarner fuelair heaters are used,each with an output of 50,000Btu./hr. Ventilating and combustionair is supplied by ram from openings

in the cannon tunnel, wing leadingedge, and external air scoops in thefuselage.

The forward heater is located aftof the nose guns and supplies hotair for defrosting transparent areasof pilot's enclosure, and heat forpilots and navigator.

Second heater extends into theleft center section wing and

supplies heat for cannoneer'scompartment and upper turret.

The rear heater is mounted aft ofthe left waist fun window andfurnishes hot air for defrosting waistand tail gun windows. Provisionshave been made for use of electricgunheaters when needed.


Center-section heating and ventilating system: (1) Heater inlet duct, (2) heaterassembly, (3) pilot's auxiliary defroster, (4) heating and ventilating air duct, (5) fuelhose line, (6) leading edge air intake scoop, and (7) heater fuel pump.


Front fuselage heating and ventilatingsystem (B-25J): (1) Heating andventilating control, (2) bombardier'sdeicer panel, (3) heating and ventilatingtube, (4) bombsight heating, ventilating,and defroster tube, (5) blower to flexibletube adapter, (6) bombardier's heatingand ventilating outlet, (7) pilot'scompartment duct, (8) duct, wing tofuselage, and (9) turret compartmentauxiliary defroster.

Rear fuselage heating andventilating system: (1) Defroster fortail gun window, (2) Avia defrostingheater for tail turret, (3) waist gunwindow defroster manifold, (4)defroster tube, (5) system control,and (6) heating and ventilatingsystem, radio compartment.


Wing DataB25H B25J

Airfoil section (root) NACA 23017 SameAirfoil section (tip) NACA 4409R SameArea, including ailerons and flaps 609.8 sq. ft. SameAspect ratio 7.48 SameChord, root 154.600” SameChord, tip 64.257” SameMean aerodynamic chord length 116.16” SameSweepback, leading edge 4° 12' 13” SameTaper ratio .415 SameIncidence, root 3° 0' 23” SameIncidence, tip 0° 30' 0” SameDihedral (L. E.) center section 4° 38' 23” SameDihedral (L. E.) outer panel 0° 21' 39” SameMaximum rib spacing,

center section 20” Sameouter panel 16” Same

Shear webs or spar locationsCenter section, front 15.52% of root chord SameCenter section, rear 67.27% of root chord SameCenter panel 33% of root chord Same

Balance FactorsB25H B25J

Gross weight c.g. location,wheels up Aft L. E. M. A. C. 232% 25.3%

Gross weight c.g. location, wheels down Aft. L. E. M. A. C. 21.2% 23.4%

Extreme forward position of c.g. possiblein flight: Aft L. E. M. A. C. 14.1% 17.3%

Extreme rearward position of c.g. possible in flight: Aft L. E. M. A. C. 28.8% 32.8%

Performance DataB25H B25J

Critical altitude 13,000ft. 14,500 ft.High speed (at crit. alt., normal power) 293 mph. 292 mph.Service ceiling 23,800 ft. 25,500 ft.Climb to 10,000 ft. 6.8 min. 6.1 min.Service ceiling (one engine) 6,600 ft. 6,900 ft.Takeoff run to clear 50 ft. 2,700 ft. 2,410 ft.Landing distance over 50 ft. obstacle 2,450 ft. 2,210 ft.Max rate of climb

(military power at sea level) 1,950 ft./min. 2,090 ft./min.

Useful LoadB25H B25J

Lb. Lb.Crew (including parachutes) 1,000 1,200Oil (40 gal. at 7.5 lb.) 300 300Oil trapped in system 60 195Fuel (434 gal. at 6 lb.) 2,604 2,604Fuel trapped in system 18 16Photographic equipment 31 31Pyrotechnic equipment 14 14Armament 1,462 580Ammunition 796 480Bombs 2,135 2,100

Useful load (normal) 8,406 7,520

WeightsB25H B25J(lb.) (lb.)

WING GROUPCenter section 1,826 1,788Outer panels 924 860Tips 16 16Ailerons 98 98Flaps 172 174

Total 3,020 2,938

TAIL GROUPStabilizer 188 196Elevators 126 126Fins 110 101Rudders 86 85

Total 510 508

BODY GROUPFuselage 2,082 1,955

LANDING GEARMain 1,578 1,562Nose 260 274Bumper 12 12

Total 1,850 1,848

NACELLE GROUPNacelles 960 914

POWER PLANT GROUPEngines as installed (2) 4,000 3,933Engine accessories 358 217Power plant controls 188 113Propellers and spinners (2) 904 973Starting system 102 90

LUBRICATING SYSTEMTanks and protection 201 200Piping, etc. 96 243

FUEL SYSTEMTanks and protection 1,200 1,023Piping, etc. 280 284

Total weight: Power plantgroup, lubricating system, and fuel system 7,329 7,080

FIXED EQUIPMENTInstruments 126 101Surface controls 390 423Hydraulic system 210 186Electrical system 612 525Communicating system (Army) 235 284Armament provisions 2,035 2,089Furnishings 484 554Antiicing equipment (defrosting tubes) 81 75

Total 4,173 4,237

UNIT WEIGHTSWing group (net area 609.8 sq. ft.)

lb. per sq. ft. 4.95 4.82Tail group (net area 223.4 sq. ft.)

lb. per sq. ft. 2.28 2.27Lubricating system per gal. of oil (75 gal.) 3.96 5.91Fuel system per gal. of fuel (974 gal.) 1.52 1.34

GROSS WEIGHTSNormal gross weight 28,330 27,000Alternate gross weights 33,047 31,737

to to36,600 35,831

Total weight empty 19,924 19,480

jl mcclellan: b-25 draft copyright 2005 p 1 of 27 · 2014. 1. 29. · packet, and messerschmitt...

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