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The Martin XB-26H Marauder was the designation given to a single TB-26G trainer that was modified to test out a new arrangement of landing gear that was being designed for the new generation of jet bombers. This was the 'bicycle' arrangement, which featured two main wheels in tandem – one in the nose and one under the rear fuselage, with small outrigger wheels in the engine nacelles. The prototype became known as the 'Middle River Stump Jumper'.
The new undercarriage arrangement was needed because the new generation of jet engines didn't have the room to carry large main wheels anywhere on their wings. In piston engined bombers such as the B-26 the engine only took up the front of the nacelle, leaving plenty of space for the wheels, but on the new jet aircraft the engine filled the nacelle. Nor was there any space in the thin wings, designed to reduce the problems of 'compressibility' that were encountered with thicker winged aircraft as they approached the speed of sound.
The first jet aircraft to be designed to use the 'bicycle' undercarriage was Martin's own XB-48. Although this never entered production the new undercarriage was used on the Boeing B-47 Stratojet, and in a modified form on the Boeing B-52 Stratofortress, which has four sets of double wheels under the fuselage (two at the front and two and the back) with outrigger wheels close to the end of the wings.