Mass Producing the A-20 Bombers

By John D Weaver
Industrial Engineer, Douglas Aircraft Co

The "How-it's-done" of Douglas Aircraft's magnificent achievement in rolling out the A-20s on its mechanized assembly lines in ever-increasing volume.


During the Battle of Britain, when Nazi bombers night-after-night rained death on London, the British turned to this plane to help repel the attackers. Originally supplied as the Boston, a medium bomber, its speed, maneuverability and load-carrying capacity were such that the RAF mounted additional guns, called it the Havoc, and sent it aloft where it proved a deadly efficient night fighter. In large numbers, it now serves the British in both versions.

With "Speed and More Speed" the battle cry of anti-Axis production lines today, a goodly part of the answer is being literally "rung" on the Douglas Aircraft Company's A-20 production lines. There the ringing of bells moves A-20 attack bombers from the stock rooms to the flight-test ramps, as regularly as the clock ticks off the minutes. And at a rate of production far greater than was ever achieved before this new, mechanized method was perfected.

These A-20s are no low-powered, lightweight planes, either. They are, in fact, all-metal fighting craft whose British versions, known as Boston and Havoc, have already chased the Axis in Libya, bombed the Axis-operated factories in the suburbs of Paris, and stalked Nazi raiders returning from the black sky over London, as night fighters and bombers.

Designed for a crew of three, the US Army Air Force's A-20 is classed as an attack bomber but is as fast as most single-seat pursuits, packing a remarkable load of bombs and ammunition, for distances no fighter can match. But, whether its specifications cause it to be labeled A-20 for the US, or Boston or Havoc for the British, it is a formidable combat plane, being produced in formidable numbers, on one of the most advanced, mechanized assembly lines in use anywhere.

Engineered under the direction of Arthur E Raymond, vice-president in charge of engineering of all Douglas plants, the first version of the A-20 type was originally accepted by the US Army Air Corps several years ago. Its original production method was "conventional" in that it was moved from jig-to-jig until finally it was capable of rolling on its own gear. But, as orders were piled upon orders, it became apparent that a faster and more efficient method, which permitted greater mechanization and more specialization by assembly line workers, could be evolved. The solution is our "vertical" assembly line.

The achievement was part of a complete streamlining of all Douglas plants. Into innumerable huddles went the team of President Donald W Douglas, F W Conant, vice-president in charge of manufacturing, T B Coulter and T H Brennan, plant layout experts, and managers of the several Douglas plants, department supervisors, and representatives of production engineering, tool engineering, and numerous other departments. Out of those huddles came assembly lines that are original in design and so well suited to their purposes that adaptations are being widely used in other plants throughout the nation. The mechanized line for production of the A-20 was one of the first that has had ample time to prove its worth.

Before A-20 fuselages are fabricated, or are even needed for final assembly of the plane, thousands of man-hours of work have already gone into the making of thousands of separate parts, exclusive of the engines, that constitute the completed plane. These have been designed, methods of making them perfected, and their raw materials specified, each in its own way, at its proper time and place, and stored so it can be conveniently transported to that place where it will be fused into the whole. Giant hydraulic presses, forming parts by the now well-known "Guerin" process, developed here at Douglas by Henry Guerin, manager of the Santa Monica plant, transform sheet stock into cut and formed parts. These, and thousands of others, whatever their size and shape, are transported through "streets" and "avenues" of the plants, by electric trains, run on regular schedules.

A Steady Stream of Salvage

Away from the presses, lathes, cutters, drilling machines and foundries goes a steady stream of "salvage" that is sorted, pressed into easy-to-handle bales, or boxed for shipment to processing plants where it is melted down again for re-use. And, all at a saving in materials and money that is impressive and important to the war effort and taxpayers' pocketbooks. In fact, hundreds of thousands of pounds worth millions of dollars are saved annually.

As do a number of other major airplane manufacturers, we prepare breakdown drawings of the subassemblies required for each type of plane. The A-20 breakdown accompanying this textnote will give an overall picture of the manufacturing problems involved. Divided roughly, the assembly might be said to come in six major steps, some proceeding simultaneously with others, and some following in sequence. Although it doesn't actually "happen" in this order, these six steps are:

  1. Assembly of the fuselage,
  2. assembling and joining inner wing sections,
  3. attaching of landing gear,
  4. mounting of the two Wright Cyclone engines,
  5. addition of the tail assembly, and
  6. attachment of the outer wing sections.

Throughout our plants, at each working "station" along the assembly lines, the production illustrations department of the engineering division has placed three-way, perspective drawings, similar to one accompanying this text, for the guidance of our craftsmen. By having before them, at all times, a clear, easy-to-understand guide, each man or team knows what is to be done and the best way to do it. In fact, the only way to do it.

Another example of this "visualizing" of the job, is our method of constructing assemblies, as far as is possible, in the same position they will have on the completed ship. Take the nose of the A-20 as an example. It is fabricated also on a vertical line where it grows, step-by-step, from its foundation piece to its final inspection, while being carried along, sidewise, at convenient working height, in the same position it will have on the plane. Because our craftsman can see it as it will appear on the plane, he finds it easier to grasp what he is doing, why it has to be done that way, and takes pride in doing his job well. Error and delay are minimized.

And, just as the nose, fuselage and other major assemblies are constructed on mechanized lines, so, too, are the inner wing section, shown in another accompanying illustration. Down the tracks of this line go jigs, each with its load rapidly taking shape, to come off at the far end, a completed inner wing section.

We at Douglas take considerable pride in the development and innovations of the Guerin process; the flush-riveting process developed here and turned over to the industry; in extensive material conservation; and are moving strenuously to exceed all expectations, high as they are, for our portions of the airplane production program. But, we are proudest of the mechanized production line technique developed here, and elsewhere in the aircraft industry.

To build A-20 fuselages we adopted the longitudinal or "half-shell" plan, whereby right and left halves are fabricated separately and, when complete with all required installations, are joined into a completed fuselage. This method offered means to construct each half, fit it speedily and handily with all interior installations, with craftsmen working on both sides of each half-shell simultaneously, and with practically no crawling into small cramped places.

The line itself is carried on two steel rails, one near the floor and one about nine feet directly above the lower rail, mounted on a sturdy framework of wood and steel that also carries electrical and compressed air outlets, light fixtures, tool racks, and such other lines as each station requires. Hanging from the top rail and running on the bottom one, are giant rectangular frames of welded, ten-inch steel tubing. Within these frames or jigs are the attachment brackets and locking pins by means of which longerons and stringers and other parts of the fuselage structure are held in place until they are locked firmly together by rivets.

Starting empty, but directly from a master jig where it has been thoroughly checked for perfect alignment, the jig picks up longerons and stringers in the first working position, gets more of them in the next, and continues to succeeding stations where the skin is clamped on and later riveted.

Because of the half-shell method, workmen can be and are stationed, at irregular intervals, down both sides of each half-shell line. They work in teams or individually without interfering with each other. Tools, such as drills or rivet guns, are permanently suspended or racked convenient to their point of use, eliminating the time formerly lost in checking them in and out of tool cribs as shifts changed, and freeing aisles of traffic jams of workmen waiting to return or obtain tools.

Directly above each position are clusters of fluorescent lighting fixtures that flood it with ample, non-glaring light.

In bins or on racks, generally behind the workman, but in some instances at his side, at the edge of the aisle but never in it, are the exact number of parts and rivets, screws, bolts, or clamps required to attach them. These were delivered, conveniently ahead of their scheduled use, by the electric train that supplies that particular station.

When the allotted time for each position's operations has elapsed, a bell rings and the entire line of jigs moves forward one position, under power furnished by an electric winch. Each move makes room for another jig to start down the assembly line. Each has come back to the starting point by way of a return track that extends from the completion end to the master jig, at the starting end.

Paralleling this fuselage "left half" line is another on which "right halves" are being simultaneously assembled. The work progresses in perfect unison, both halves reaching the "end of the line", or approximately the halfway point, at the same time. There, with longerons and stringers and skin completely riveted, and initial installations also in place, both halves are removed from their respective jigs for the first time.

Overhead cranes pick up the half-shells, give them a quarter turn, and deposit them in separate dollies that are a convenient working distance apart, but hold the half-shells with their inner surfaces facing each other, ready to be joined. On these dollies the half-shells continue on down the line, but now moving sidewise instead of endwise as they started. Interior installations are completed in this position, at several stations and the two halves are then simply and speedily joined.

Relieved of their half-shell loads, the original jigs are picked up by overhead cranes and swung over to the return track where they are towed back to the master jig for a complete check of their accuracy, stopping en route if necessary, for any repairs or adjustments that may be needed.

The completed and joined fuselage next acquires its two inner-wing sections. These were assembled on another, horizontal line of unusual design. On this "wing track" inner-wing sections start as three subassemblies that are joined in a stationary jig. When the necessary mountings are attached, this beginning assembly is removed from the jig and enters the wing track assembly line.

This track, developed here at Douglas and now adapted elsewhere in the industry, holds the inner wing sections, rights and lefts alternating down its entire length, at a convenient working height. At each station additional parts are joined until, when each assembly reaches the far end, it is ready to be swung by overhead crane, into position for mounting on the fuselage.

As the fuselage and inner wing sections are joined, landing gear is attached, and then engines and tail assembly are set in place. The rapidly shaping ship is now rolling down another assembly line over which a huge spray booth has been erected. As it passes through this booth it acquires its camouflaged finish. Then continues to roll, on tracks, to the final assembly.

At the beginning of the final assembly, outer wings are attached and the virtually completed plane is switched to one of two final assembly tracks where it rolls by gravity, on its own steerable front wheel and dollies, in a diagonal position, to the far end where it leaves the line a completed A-20 type bomber, ready for its check flight.

This final assembly line is believed to be the only one that harnesses the force of gravity, saving both manpower and mechanical power. A gently sloping floor provides free motive power for the movement of each plane from station-to-station during final assembly. Simple but effective brakes stop and hold each plane at each station, as bomb racks, cockpit hatches, and other assemblies are added.

Here, too, numerous tests are made, among them that of the hydraulic controls, retraction mechanism of the landing gear, and final checks by the US Army Air Forces' inspectors.

On such production lines then, the A-20s and their Boston and Havoc relatives are born and given the unsurpassed stamina, speed and maneuverability that prompted Lieut Gen Henry H Arnold, chief of the United States Army Air Forces, to wire Douglas employees recently, as follows:

"Douglas bombers have smashed at the enemy again. Four Bostons, piloted by Allied airmen, inside a pioneer daylight raid over the Libyan desert on the Nazi airbase at Martuba in North Africa. Diving on their targets, they sent hangars up in flames. Teaming up with American made fighter planes, they then bagged four German pursuit ships."

Perhaps before this can be published A-20 kin of those Bostons will be demonstrating to the Axis that being born "on the half-shell" is no handicap.

This article was originally published in the June, 1942, issue of Aviation magazine, vol 41, no 6, pp 86-89, 91.
The original article includes 15 photos and one detail drawing.
Photos are not credited, but are certainly from Douglas.
The breakdown drawing below was not included in the article as printed. This drawing, taken from the "Sketchbook of Design Detail" column in the February, 1943, Aviation, p 153, is included to illustrate the main subassemblies of the A-20.

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