The Bell P-39 Airacobra was one of the principal fighter aircraft in service with American forces at the start of World War II. (The P-39 was at first for a short time designated XP-45.) The aircraft's unusual design featured its Allison engine mounted in the middle of the fuselage, just behind the pilot, driving the propeller through a driveshaft passing between the pilot's feet. The purpose of this was to free up space for the heavy main armament, a 37 mm T9 cannon firing through the center of the propeller hub for optimum accuracy and stability when firing. The weight distribution necessitated a tricycle undercarriage, a first among American fighters. Entry to the cockpit was through a side door rather than a moving canopy. The weight distribution of the P-39 supposedly is the reason for its tendency to enter a dangerous flat spin ? a characteristic Soviet tests proved to the then-skeptical manufacturer who had been unable to reproduce them.