Producing the B-25 Bomber

By Robert E Dawe
Factory Manager (Inglewood Div),
North American Aviation, Inc

North American is one of the leaders in aircraft quantity production and the reason is simplicity and flexibility in its production lines

The B-25 Mitchell bomber is already a veteran among the combat planes of the Allied Nations air forces. It is widely known, of course, as the ship Maj Gen James H Doolittle used to bomb Tokyo. And it is no secret, among Air Forces personnel, that it has been used for high and low altitude precision bombing, troop strafing, night fighting, torpedo launching, express and personnel transport, reconnaissance, and long-range pursuit work.

The B-25's tricycle landing gear contributes to its ability to land on small, rough runways, and its short takeoff run permits it to get off of them with ease. On the ground it has unusual stability. But in the air it is officially credited with speed and maneuverability close to that of pursuit planes. These performance features explain many of the uses to which it is put, beyond what is normally expected of a good medium bomber.

At the same time that North American's engineering staff was designing the B-25 for combat flexibility, it also designed the plane for production flexibility in close cooperation with the tool design department. We who are concerned with production at North American like to feel that we have contributed somewhat to the combat versatility of the B-25 bomber through our maintenance of flexible production methods in the plant, which in turn has enabled us to incorporate required design changes quickly either for general improvement of the airplane from a tactical standpoint or for performance of specific missions under special conditions.

Fabrication Breakdown

The B-25 Mitchell is built according to a component breakdown plan in which such components as the fuselage, start as side, top and bottom panels of three main sections. After tie-together assembly in stationary jigs these panels are clamped into castered jigs for the trip through numerous fabrication and installation stations to the master jig. To each panel is attached all of the tubing, wiring, cables, radio brackets, switch boxes, interphone boxes, and radio equipment it is possible to install at that stage of completion. This timing enables our men and women to do this work while standing in normal positions, with sides, floors, and ceilings held in an upright position and readily accessible. Considerable time and energy is thus conserved and production thereby expedited.

Four lines of castered, mobile jigs carry top, bottom, right and left side panels of the front section of the fuselage from the tie-together, (starting) jigs through six stations to the front master jig. Four other similar lines carry like panels of the rear section through six stations to the rear master jig. Both master jigs are contained with a single steel framework, of the one-story and balcony type, where work is done above and below center lines simultaneously. Both jigs feed directly into overhead-carrier fabrication and installation lines which parallel each other, starting at opposite ends of the master jig structure, to the final assembly line, but at right angles to it.

Overhead Conveyors

In the master jigs bulkheads are clamped in first, with quick-acting clamps. Next, the bottom, sides and top panels are joined to the bulkheads. Structural and joining riveting proceeds in the master jigs until it is about seventy-five percent complete. Then overhead cranes lift the sections from the jigs and lower them to cradles on the floor. Here carriers are attached and the sections are raised until the carriers enter the overhead tracks of the system where front sections proceed sidewise and rear sections lengthwise, their tail ends forward, to the end of the lines immediately adjacent to the final assembly line.

On the overhead lines fluorescent work-lights are hung at convenient points in each section. They are not removed until the section is completed. Electric current, for both lights and tools, and compressed air lines, are carried in trolleys above the sections, where they are readily accessible but not in the way. Work stands attach between the carriers where they tie the line of carriers into a. continuous train, ride free of the floor, and are detached for return to the starting point only when work is finished in the last station of the line.

Final structural riveting is completed during passage of the sections through the first three stations of these lines, but installations proceed from the first station on to the last. Because of a greater number of installations, front sections pass through forty-one stations while rear sections are being completed in nine.

At the three-quarter point, the bombardier nose section arrives from a separate assembly line, transported on a castered cradle, and is attached to the front section. The enclosure over the pilot's compartment, transported on an overhead carrier, also arrives at this point and is joined to the front section.

Parts for all sections, throughout the plant, are fed from fabrication stocks to conveniently located racks, bins, and shelves adjacent to the stations where they are needed. Workers obtain parts from them as needed, without formality, by merely taking them out themselves. Material Control had assigned them and checked them out before they were transported, by aisle trains, to the small working station storage places.

A feature of the carriers on the fuselage front and rear section overhead lines consists of built-in winches for lowering them for joining to the center section on the final assembly line. After sections are thus removed, overhead carriers are run off the track end, and on to a track-high carrier on which they are taken to the head of the line where they are again attached to front and rear sections just lifted from the master jigs.

Between the front and rear section lines, and flowing in the same direction, is the engine line. This, too, is of the overhead type. Power plants move through the numerous stations of this line as they acquire mounts, special equipment, wiring assemblies, and controls. From the final inspection station they roll across an aisle and are in position for attaching of the mounts to the nacelles facing them.

Center sections are assembled on a separate, mobile, powered line that starts from the tie-together jigs and merges with the rear section and engine lines in a combination turntable and elevator. Landing gear also is attached in this combination station and the fuselage is now virtually complete and has inner wing sections in place, as they were assembled as part of the center section.

Separate Center Section Line

Center sections, composed of prefabricated side panels, engine nacelles, wing compartments, dividers, ribs, and sections of skin are tied together in a series of stationary jigs. As they emerge from the last of these they are attached to carriers of an overhead track system and begin their steady progress through 16 fabrication and 12 final assembly stations where structural riveting and installations are completed. The line moves at intervals, through power supplied by an overhead dragline.

Along this line apply and drill jigs and heavy tools are suspended and counterbalanced to save time, space and breakage. This practice, long in use, is proving of real advantage as more and more women take their places along the lines. Counterbalancing permits them to work with heavy apply and drill jigs that would otherwise prevent their employment for such jobs.

As the section enters the line a small rack or board is attached to the left side for the clipboards on which "Sign Off Sheets", "Mechanical Squawks", "Electrical Squawks" and "Shortage Sheets" are clipped. As the section proceeds down the line, workers and inspectors enter their reports on these sheets, the pads of "Squawks" remaining with the section until the plane, of which it will become a part, is completed and "sold" to the Air Forces.

Joining Main Subassemblies

Fuel tanks, nacelles, wing sections and other major components of the center section are assembled in jigs in departments grouped about the head of the center section line. They feed their output toward tie-together jigs nearer to the starting point of the assembly line. Installations begin in the component assembly departments and continue throughout the 28 stations of the center section line.

About three-quarters of the way down the final line, the front section arrives, transported by the overhead carriers which suspend it on its final assembly line. Here, the front section is lowered to a cradle, casters of which run in tracks, which shuttles the front section into position for joining to the center section. When the two sections are joined, a carrier of the center section overhead line is attached to the front section, for additional support, and the shuttle-cradle is freed to return to pick up another front section and shuttle it into position for joining to the next center section on the line.

Overhead Turntable

An ingenious floor-saver stands astride the end of the center section line. It is a combination turn-table and elevator which is also a working station. The now combined front and center sections leaves the powered portion of the center section line and are rolled forward by hand on to the overhead rails of the turntable elevator. There they are turned 90°, with nacelles facing the end of the engine line. The engines, which flow off of the end of a previously mentioned installation line directly into the turntable-elevator, are installed and the assembly is rotated 180° in the turntable-elevator, is elevated so a floor carrier of the final assembly line can be run under and be attached. A fuselage rear section then rolls forward and is joined.

When ready to leave the turntable- elevator station, landing gear will have been attached and the overhead carriers removed. Mounted on the floor carrier the now completely joined fuselage- inner wing assembly rolls nose forward in a 90° arc and starts down the first of four legs of the final assembly line.

Drag Line Conveyor in
Final Assembly Lines

Before the remaining joining, installation, and painting operations are completed, the ship reverses its direction three times, traveling the length of the final assembly line four times, to emerge from the master paint booth onto the final check ramp where it receives preflight tests and tune-up.

Carriers used on the final assembly line are triangular-shaped platforms mounted on two grooved, small wheels at the base or wide end, and on two plain casters at the apex end. At those portions of the line where lateral direction is required, the grooved wheels run in U-shaped steel channels or on flat steel "rails". On the straight-away portions of the line they run on inverted V-shaped channel iron rails which fit the V-shaped grooves in the wheels. The apex wheels run on the floor when the carrier direction is straight and between flat steel bands or in U-shaped steel channels when it is necessary to guide them.

Power for either interval or continuous movement is applied to the line through draglines which slide along just above the floor, centered between the rails. They are inverted U-shaped channel steel, and fit over smaller, similar channels anchored to the floor as guides. Power is transmitted to the channel dragline by a standard chain dragline under the first two stations of each leg of the four-leg line.

For forward movement, dogs on the channel dragline engage pawls on the steel frames of the carriers, pushing the carriers forward continuously or intermittently, as desired, the length of one working station. Then. the channels are pulled back until the dogs pass under the pawls of the next carriers and are in position to re-engage them. At that point the dragline stops and reverses its direction, pushing the line of carriers forward again.

The twin-tail empennage is assembled from prefabricated sections, on a line housed in a balcony above a stockroom section of the main floor. Complete tail assemblies are transported by traveling overhead cranes from the balcony storage space to a position directly over the first station of the final assembly line. There the empennage is lowered and bolted into place. Jigs in which tails are assembled are of an unusually flexible design that permit their being turned easily from vertical to horizontal positions as the work requires.

Outer wings are assembled in stationary jigs, in two major components, the leading edge and the trailing edge. When firmly tied together these components are removed to castered jigs, running on the floor with casters guided by parallel strips of steel band attached to the concrete. Leading edge sections pass through 14 stations and trailing edge sections through 6 stations, to reach the master wing jig. Here the two components are tied together and are about 70 percent completed before being removed and attached to carriers of another overhead line. On this line, which moves continuously and doubles back to its starting point, outer wing panels, rights and lefts alternating, are completed.

A feature of this overhead line that reduces worker fatigue and expedites production, is varying track levels. Between several of the 27 stations on this line the track rises or drops to raise or lower the sections to the most convenient working height.

Another feature, designed to eliminate confusion, is suspension of electric power and compressed air lines between the two rails of the overhead track, on reels with spring rewind. Spaced so each station has whatever number of lines it needs, with just enough unwound to reach the work, this feature keeps the floor from becoming cluttered and lines out of the way of workers.

Upon completion, the outer wing sections are transported in cradles to the last station of the second leg of the final assembly line, where they are attached sufficiently for the swing-around of the ship into the third leg. On this third leg joining is completed and all connections of tubing, wiring, and cables are made.

Flexibility in equipment for doing the work along the four legs of the final assembly line is well illustrated by the stands and platforms which surround each ship. These are of tubular steel framework with wood floors, mounted on swivel or fixed direction casters which are guided by channel iron tracks. Main platforms are built for a particular working point and are joined to the plane carrier by eyebolts and pins. Other stands are added as the work requires, and attach to the carriers or to the main platforms. They are attached and detached in a few seconds, but while in place roll with the ship. Throughout the plant, whether in fabrication or assembly departments, arrangements, machines, tooling, material flow — in fact everything — is as flexible as it is humanly possible to make it. Just as war requirements prevent rigid freezing of designs, so also do they forbid rigidity in manufacturing methods.

To adhere to the flexibility rule it is necessary, among other things, to keep a rather large plant-maintenance department busy moving equipment, assembly lines, stock rooms, and the hundreds of other essentials of a big manufacturing plant. Since this article was first requested by Aviation, for example, preparation of it has been postponed twice because of major changes in the B-25 lines. Each change brought still greater efficiency, or made possible changes in the planes themselves that increased their effectiveness.

This article was originally published in the March, 1943, issue of Aviation magazine, vol 42, no 3, pp 106-111, 359, 361-362.
The original article includes 17 photos of the B-25 assembly line and a full-page flow diagram of the B-25 assembly sequence.
Photos are not credited but are certainly from North American.

Photo captions: