Although the Bell P-39 is often thought of as a premier fighter in the hands of Soviet pilots, it is also well remembered as an airplane that was almost a great fighter. So great was the Soviet success with the P-39, some forget that the aircraft also served well for the British, Free French, and US. Created in response to the US Army Air Corps Circular Proposal X-609, issued in February 1937, Bell Aircraft's P-39 boasted an innovative layout. The engine was positioned behind the pilot with a long shaft connecting it to the propeller. Lack of a turbo-supercharger and limited room for on-board fuel tanks made the aircraft ill-suited to high-altitude combat, and as a result many of the P-39s were shipped to the Soviet Union, where relatively low speeds and low altitudes were characteristic of air combat. Soviet pilots appreciated the P-39's armament, particularly its 37mm nose cannon. Despite its high-altitude limitations, US combat forces made good use of the P-39 in the Mediterranean and Pacific theaters, where in lower-altitude engagements, the aircraft held its own against Zeros and other enemy planes.