From 1943, the B‑26 appeared in four major variant series, all known by the general type name of “Marauder” – a name given to the aircraft by the RAF, which, in October 1941, had ordered 52 later production B‑26As, operating them as the Marauder I.

The first B‑26s to see combat with the USAAF in Europe were of the B-4 type, which introduced a longer nosewheel leg to give the aircraft a higher wing incidence to reduce its take-off run, four forward-firing fuselage package guns for use by the pilot, and provision for a pair of 0.50-cal. machine guns in the rear fuselage hatches. Produced at Middle River, the B‑model can be considered the “main” variant, being built in several production blocks, each incorporating successive improvement. The first B‑model came off the assembly line in May 1942 fitted with Pratt & Whitney R‑2800‑41 engines. These were two-row, 18-cylinder, air-cooled radial designs, displacing 2,804 cubic inches, with a bore of 5.75in. and a stroke of 6in. They produced 2,000hp at take-off and 1,600hp at 13,500ft.

Maximum speed at 14,500ft was 317mph, but weight increased to 28,000lb empty and 34,000lb fully loaded, including 3,000lb of bombs. The R-2800 also incorporated one of the most advanced baffle systems to control cooling air flow, and at the time of its introduction no other air-cooled engine and only a few inline water-cooled engines could match the R-2800’s power-to-weight ratio. The engines drove Curtiss electric propellers, with the angle of the blades controlled while in flight to provide maximum efficiency and to maintain constant engine speed under varying operating conditions.

The B-1 saw the spinners removed, new engine cowlings with enlarged air intakes, additional armor, revised twin tail guns and a 28in. extension to the fuselage. Following a run of 641 B-1s to B-5s, all of which featured progressive minor improvements, the next main variant was the B-10. This became recognized as the “long-wing model,” with its wingspan increased by six feet to 71ft in an attempt to reduce wing-loading.

The aircraft was 58ft 3in. in length, 21ft 6in. in height, and had a wing area of 658sq ft. As the December 1944 manual stated:

The B‑26 has a high wing loading. It can take plenty of punishment, but you must know and respect its tolerances. It wasn’t built for aerobatics, inverted flight, or violent stall maneuvers. It is not a pursuit-type airplane.

Armament comprised a maximum of 12 0.50-cal. Browning AN/M2 machine guns, including, in addition to the flexible nose gun used by the bombardier, a fixed gun in the starboard side of the nose section. Like the twin forward-firing package guns mounted on either side of the lower curve of the forward fuselage and intended for strafing, this weapon was fired from the pilot’s control column. However, the fixed gun made the nose area crowded, and so was subsequently removed in the ensuing B‑45.

A Martin 250CE electrically powered dorsal turret was located forward of the waist gun hatches and just aft of the rear bomb-bay. It was designed as a “drop-in” unit and was “hung” from the upper fuselage, rotating continuously in azimuth to elevate the two 0.50-cal. machine guns from five degrees below to 85 degrees above the horizontal. Ammunition was fed to the guns by two automatic electric booster motors from two 400-round boxes in the forward part of the turret. The profile of the aircraft was protected from gunfire by an interrupter which was interwired with the firing mechanism. Hand grips with built-in gun triggers controlled the turret’s rotation and gun elevation. Also located on the grips were a microphone and a high-speed switch. Connections for heated flying suits, oxygen, and the interphone ran into the turret through a swivel fitting.

Master switches for azimuth and elevation power were under the gunner’s seat. A camera, gunsight rheostat, and gun safety switches were installed to the right of the seat.

The B-10 received mechanically improved R‑2800‑41 engines, although there was no increase in horsepower. The tail was increased in height and area to improve yaw stability. The B‑model series blocks ran through to the B-55. Again, interim blocks incorporated changes and/or refinements. The B‑15 saw changes to the oxygen system and was the last variant to have the “stepped” tail gun position with its two hand-held weapons. The B-20 had further changes to the oxygen system, a shorter chord rudder and, starting with the B‑26B-20 and B‑26C-20, a Bell Type M-6 powered tail “stinger” with two 0.50-cal. machine guns along with armor plate for the gunner. The guns, fitted with G-11 solenoids, were mounted side-by-side.

The Bell Sundstrand system installed in the M-6, consisting of two hydraulic pumps and a motor in one unit, azimuth and elevation vanes, and a control unit mounted on the back of the armor plate, was designed to provide remote control of gun movement. Gun charging handles were installed in the most forward position on each gun, and they were accessible through the armor plate doors. The firing triggers were incorporated in the gun mount control grips. Aiming of the guns was undertaken using an N-8 illuminated sight mounted immediately forward of the window in the armor plating.

The B‑26B‑25 had its rear bomb-bay sealed to prevent overloading and the B-55 had an improved bombsight. The B‑26C, which appeared in mid-1942, covered all of the models produced at the Omaha plant through to the C-45, which saw the end of production in the spring of 1944. The F-series referred to the “twist-wing” model which had a seven-degree angle of incidence, appearing after the F-6. This wing modification was again intended to shorten the take-off run, although maximum speed was compromised. However, only the B‑26F-1 was delivered to the USAAF. The last production model, the B‑26G, differed only slightly from the F. Aside from being powered by Pratt & Whitney R-2800-43 engines, the aim of the G-model was to incorporate standard equipment used by both the USAAF and US Navy wherever possible to aid in the interchangeability of parts. There were eight principal B‑26G blocks (G-1, G-5, G-10, G-11, G-15, G-20, G-21, and G-25). Early examples were fitted with a “blast blanket” intended to protect the structure of the forward fuselage around the package guns, as well as modifications to the fuel and hydraulic lines and an emergency landing system. Other blocks had changes to the portable oxygen and radio systems.

The main fuel tanks in the B-, C-, and early F-models each held 360 gallons and were installed in the wings, inboard of the nacelles. The outboard auxiliary tanks each held 121 gallons. Fuel was supplied to the engines exclusively from the main tanks. In the later F- and G-models, the capacity increased to 380 gallons for the main tanks and a gallon less in the auxiliaries.

The hydraulics in the B‑26 featured an open system of the accumulator-pressure regulator type, meaning that hydraulic fluid was supplied from the main hydraulic reservoir to two engine-driven pumps under atmospheric pressure.

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