The main purpose of this configuration was to free up space for the heavy main armament, a 37 mm Oldsmobile T9 cannon firing through the center of the propeller hub for optimum accuracy and stability when firing. In fact, the entire design was made to accommodate this gun in the aircraft. This happened because H.M. Poyer, designer for project leader Robert Woods, was really impressed by the power of this weapon, and he pressed to design an aircraft made with this weapon, though the original concept had been a gun of 20-25 mm mounted in a conventional manner in the nose. This was unusual, because the fighters had always been designed around the engine, not a weapon system. Although devastating when it worked, the T9 had very limited ammunition, a low rate of fire, and was prone to jamming.
A secondary benefit of the mid-engine arrangement was to create a smooth and streamlined nose profile. The weight distribution necessitated a tricycle undercarriage, a first among American fighters. Entry to the cockpit was through side doors (mounted on both sides of the cockpit) rather than a sliding canopy. Its unusual engine location, and the driveshaft, caused some pilot concern at first, but experience showed this was no more of a hazard in a crash landing than with an engine located forward of the cockpit, while there were no problems with propshaft failure.
The Bell P-39 Airacobra was one of the principal American fighter aircraft in service at the start of World War II. Although its mid-engine placement was innovative, the P-39 design was handicapped by the lack of an efficient turbo-supercharger, limiting it to low-altitude work, although the type was used with great success by the Soviet Air Force. Together with the derivative P-63 Kingcobra, these aircraft would be the most successful mass-produced, fixed-wing aircraft manufactured by Bell.
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