The Martin Marauder B-26

by Roger Ward and Phelps F Mace

The B-26 Marauder, currently playing a key role in the invasion of both north and south Europe with its pinpoint bombing and record low loss in combat (currently under 0.4%), has had a long and varied career from an engineering standpoint. Many innovations, which have since become standard for fast, hard-hitting combat aircraft, were incorporated as revolutionary items back in the design stage of 1939 and 1940.

It was on January 25, 1939, that the Army Air Forces requested American aircraft manufacturers to design a medium bomber capable of a speed from 250 to 350 miles an hour; range of 3,000 miles; service ceiling of from 20,000 to 30,000 feet; a maximum bomb load; a crew of five; and armament consisting of four .30-caliber machine guns. High speed was the prime requisite. Takeoff and landing speeds were practically neglected in the final evaluation of the design. In brief, it was to be a medium bomber with pursuit-ship speed.

On July 5 of that year the Glenn L Martin Company submitted to an Army Board, headed by Brig Gen Jacob Fickel, at Wright Field, a Detail Specification, which outlined the guaranteed performance by the company engineers. Also a Proposal was submitted, ie, an offer to build an assigned number of bombers in a specified period. The Martin Company won the Army's bid by the Method of Evaluation, based on 1,000 points for perfect score, by a margin of 140 points over its nearest competitor.

Time was of the essence. No prototype could be built. Therefore the bugs that crop up in all prototypes could not be ironed out before production began. The first Marauder was a production model engineered and built on production fixtures and using production tools throughout.

The general design scheme was that of a low-drag profile rotated. Years before this design had been advanced for the construction of a radical-type wing. The result traced out a symmetrically perfect fuselage and nacelle with very low drag. The inside diameter of the fuselage was sixty-seven inches, the same as the width at the point where the pilot can see the wing tips past the propellers.

To attain the requisite speed, a wing loading somewhat higher than had been considered normal in previous designs was used. A front bomb bay, almost identical to that of the B-17, was installed and an extra one was added to the rear to account for the highest maximum bomb capacity. Before the first bomber left final assembly the original armament was increased greatly and .50-calibers were installed in the tail and in a deck turret, which was power operated in azimuth and manually operated in elevation.

In order to achieve a satisfactory production design, the entire airplane was broken down into about 600 minor sub-assemblies and thence to about 30 major assemblies, all readily manufactured and completed in detail prior to final assembly.

The general production setup called for extensive use of spotwelding in secondary structure to eliminate, where possible, the more costly and time-consuming process of riveting. The stretch press, used by the automobile industry at the time of World War I to form fenders, was reintroduced to shape convex-curved pieces of aluminum skin, four times the average size in other aircraft. These large sheets of skin were employed in the fuselage and cowling to eliminate the multiplicity of stringers, rivets and other small detail parts normally associated with a fuselage of this size and length. Hundreds of castings, forgings and plastic parts were used to simplify mass production methods and eliminate the use of critical materials. Thus, the B-26 uses more castings, forgings and other miscellaneous parts adapted to intensive production methods, fewer rivets and detail parts, than any comparable airplane. Some of the new design elements in the Marauder follow:

WING ARRANGEMENT — It was decided to depart from the usual procedure of keeping wings integral from nacelle to nacelle by breaking the wing panels at the sides of the fuselage. This entailed the eventual design of the "dumbbell" lower spar chord forging to eliminate the use of heavy and costly fittings. Manufacture of this forging, the largest aluminum type ever made, required five sets of dies, each weighing more than two tons. Thus the wing and its engine and nacelle is an entire assembly. The center section was eliminated. Each wing is attached to the dural forgings at the wing roots, which serve as anchors for the attaching bolts. This arrangement facilitates assembly and replacement.

FUSELAGE — Its construction around a very strong keel made for almost perfect streamlining, and extra strength in crash landings. B-26 crews nearly, always walk away from a forced landing. This same keel was also excellent as an anchor for the external torpedo rack. There are no stringers, just skin and forming frames — five compartments in three major sections.

NOSE — The Plexiglas nose, a double contour affair with an upper and lower shell, evolved after about a year's cooperative research by Rohm and Haas and the Martin Company. The two shells, cemented together at the center rim, formed a nose section about 33" long and 48½" diameter. It was fastened with tension bolts to the fuselage frame with a cork rubber seal in between to cushion the nose and allow for expansion and contraction due to weather. Noses previously had been constructed of far smaller strips of flat resin material.

TURRET DOME — This is one of the largest single castings ever used in the field of resins. It has a 41¼" diameter and 18" height.

WINDSHIELD — The curved glass windshield was likewise the subject of considerable research on the part of the Pittsburgh Plate Glass Company and required an entirely new technique of forming and manufacturing. Parts of a cone are glued together by laminated glass with a vinyl bond, thereby retaining many straight line elements.

AUXILIARY PROPULSION — The Marauder was the first aircraft to have auxiliary efflux propulsion. This is a jet-like setup whereby exhaust gas energy is utilized to add to the B-26's speed by about 5 mph.

ARMAMENT — The necessity for a fully power-operated turret with twin .50s became apparent after six months of design The first turret was designed, built and test fired at Aberdeen Proving Grounds in twenty-one days. As a result these turrets were installed on all ships at Langley Field, home of the first B-26 group to get into combat. They went out December 8, 1941, to the Southwest Pacific. The deck turret, now used on many types of aircraft including B-24s, B-17s (a few), A-30s, A-20s, PV1s and PV-2s, was the granddaddy of all twin .50-calibre operated turrets in the AAF today.

The tail gun emplacement was subject for considerable research to secure an arrangement to permit large firing angles; a seating or kneeling position for the gunner; and to streamline the aft end to avoid excessive drag and loss of speed. Eventually a twin .50-calibre arrangement was evolved which utilized remote feeds through specially designed ammunition tracks, the first of its kind to be employed by the AAF. The latest evolution of this emplacement, designed by Martin and Bell and built by the latter, uses a remote control power-driven unit. The gunner is wrapped in an armor-steel bulkhead, affording him maximum protection, and peers through an eye-level window of three-inch thick laminated glass. This is proof against some of the heaviest types of enemy shell-fire. The ammunition supply is fed from containers in the aft bomb bay, instead of from drums located in the tail emplacement proper. This arrangement permits an almost unlimited supply of ammunition. The cartridges reach the tail position by a roller-track, riding upright instead of horizontally as in previous systems. The new vertical scheme eliminated much of the old difficulty of ammunition jamming in the tracks and also reduced to a minimum the power needed to move the cartridges to the guns.

LANDING GEAR — The main landing gear in the tricycle gear arrangement had a unique means of retracting. Instead of retracting aft, forward, outboard or inboard, the gear retracted upward, by means of a W-strut and shock strut scheme. Double landing strength results for a bomber that lands at a high speed. The gear can withstand a landing impact of 70 tons from this airplane, currently about 19 tons in weight, ready for battle. Gravity aids the gear in returning to landing position.

FUEL TANKS — It should also be mentioned that the original B-26s were the first Army airplanes to be completely equipped with self-sealing fuel tanks and self-sealing fuel lines. The original contract had provided for the use of Mareng non-self-sealing rubber cells, but it was a relatively simple matter to convert to the self-sealing.

BOMB SHACKLES — The electric bomb shackles used on the original B-26 were the first used by the Air Corps. Development work on these units and the interest aroused by the Martin Company's effort to improve the older types of bomb shackle equipment (used on the B-10 and B-18s), led to a research program by Wright Field which eventually resulted in the adoption of a Standard electrical-release unit for use on all Army aircraft.

BRAKES — The B-26 was the first airplane to use emergency air brakes. The air brake system was developed just before the ship flew, when it became apparent that without brakes it would require approximately two to three miles to coast to a stop. The system used on the early B-26s was invented, designed and installed in less than twenty-four hours. Today vastly improved air-brake systems are in use which give the pilot the same feel and control as standard hydraulic type brakes. The B-26 was also the first Army airplane to employ fluorescent lighting of instrument panels, eliminating the use of individually lighted instruments.

All these problems and a great many more were solved during the nine months that it took to engineer and fly the first ship.

It was a tense moment on November 25, 1940, when the first Marauder rolled out of final assembly doors. The flight was uneventful and lasted approximately one-half hour. It indicated the necessity for improving the slightly overbalanced rudder. An excessive amount of aerodynamic balance had been employed in the design in an effort to achieve low stick forces at high speed. In view of the fact that there were a great many airplanes in final assembly which were badly needed by the Air Corps, it was decided to make no major changes in the control surfaces but to heavy-up the controls by reversing the direction of travel of the trim tabs.

The results of these flights were better than the B-26s most ardent admirer had dared hope. Turned over to the Army, it then underwent an "accelerated service test."

On February 25, 1941, the flow of production planes began and has continued ever since. Currently used in combat by the Army Air Forces, the Navy, the RAF, it has fought the world over. It has also been flown by the French, the Canadians, the South Africans, the Australians and the Greeks.

The Marauder has proven itself the safest Allied bomber in Europe and had a loss-in-combat record of less than one-half of one per cent at the end of its first year of operations over France, Holland and Belgium on July 17, 1944. Through the war in Europe, the Mediterranean and the Southwest Pacific — as well as at Midway, Alaska and the Aleutians — the Marauder has proven one of the most versatile of Allied serial weapons. B-26s were used with fragmentation bombardment at Buna; strafing at Salamaua and along the coast of Normandy; torpedoes at Midway; depth charges at Kiska; as escort fighters of Portugal; as RAF patrol bombers in the Mediterranean; as skip bombers at Mt Soputan; as long-range bombers over Rome; as pinpoint bombers over Rome, Florence, Chartres, Naples, Paris; as heavy bombers over the French invasion coast and Sicily; as attack bombers at Kasserine Pass; and as cargo planes at Guadalcanal.

The day after Pearl Harbor, a group of short-winged (65-foot span) Marauders with veteran crews left Langley Field, VA, and flew the Pacific to General MacArthur's aid — the first organized group of twin-engine planes to arrive in Australia. For the next eight months this group fought the Japs in New Guinea, helping to save Australia, and checked the Jap invasion threat to the south while shooting down 96 Zeros to six Marauders lost in combat. At Midway and in the Aleutians, serving as torpedo planes, Marauders damaged three Jap carriers, sank a cruiser, four destroyers, as well as numerous transport and cargo vessels. Only two Marauders were lost to enemy ack-ack in the Aleutians in all 1942 operations: the two Marauders lost at Midway were a fair trade for two Jap carriers seriously damaged, perhaps sunk.

In North Africa, Sicily, Pantellera, Lampedusa, Sardinia, Corsica, southern France and Italy, the Marauder served effectively. It participated in the first two Rome raids on July 19 and August 13, 1943, and followed with many others. In one two-day period over Salerno in August, 1943, Marauders downed 53 enemy fighters. Only five B-26s were lost. During the first 17 months after the invasion of North Africa, Marauders flew 15,000 sorties; dropped 18,000 tons of bombs on enemy installations; suffered 150 losses. many of which came early when B-26s were unescorted; downed 270 enemy fighters in aerial combat, destroyed 400 on the ground and damaged 300. They were commended by Lt Gen Ira Eaker, commanding the Mediterranean Allied Air Force for "stealing the air show" at Cassino.

But the biggest operations were in Europe, operating from bases in England against targets along the invasion coast of France, Holland and Belgium. There Marauders of the Eighth Air Force, and later the Ninth, flew 29,000 sorties during the first year, ending July 17, 1944; they dropped 52,000 tons of bombs, while suffering only 139 losses. The first combat aircraft to hammer the invasion coast on D-Day, June 6, 1944, they have since been used in close support of the ground forces during their rapid advance to and beyond Paris.

Dimensions — span 71' (the present version has enlarged span); length 58' 2½"; height 20'; wing area 650 sq ft.

This article was originally published in the January, 1945, issue of Air Tech magazine, vol 6, no 1, pp 17-27, 68.
The original article includes 9 schematic diagrams, a three-view drawing, an exploded-view subassembly drawing and 9 photos.
Photos credited to The Glenn L Martin Co.

Schematic diagrams:

Note: diagrams have been altered: changed to black-on-white and tables moved. —JLM
Photo captions:
    uncaptioned inflight view from 10 o'clock high
  1. First B-26 off the line was accepted as standard. No X or Y models were ever constructed.
  2. .50-cal gun replaced flexible .30-cal formerly installed in nose
  3. First combat type in action against Japs, B-26A with 2,000-hp P & Ws hit 427 mph speed.
  4. Revised tail position with flexible twin .50-cal guns increased B-26B armament to five fifties. Overloading reduced maximum speed, led to design of B-26C having wing span upped to 71 ft.
  5. Late B-26B mounted two package guns on each side of fuselage, standard waist armament of two .50-calibers. Single fixed fifty was added to flexible gun in nose. Note battered condition of plane which had run-in with enemy fighters and flak.
  6. On English airfield, 500-lb bombs are set for loading into Marauder. Externally identical to B-26B, this model incorporated many interior improvements.
  7. &
  8. Latest types, B-26F and B-26G carry two-gun power turret in tail, improving offensive and defensive capabilities. Total armament remains at twelve .50s.
Note: photo captions have been relocated to individual photos. —JLM