At the beginning of WWII, the P-39 was one of the principal fighter aircraft of the US Army Air Corps. It was unusual in the fact that the engine, an Allison 12 cylinder in line, water cooled engine, was installed behind the pilot and drove a propeller shaft that was located between the pilot's legs. This allowed a large of armament to be located in the nose, including a 37mm cannon. In addition, it was the first US fighter that had a tricycle landing gear arrangement rather than a tail wheel. The initial version was equipped with a 2 stage turbosupercharger, which gave it a very high top speed at the time. Britain made a large order, but stipulated the design without the turbosupercharger. This severely hampered its performance at high altitude. The British were disappointed at the high altitude performance and cancelled the order. The US kept a large percentage of the order, designating them as P-400's. The P-39 was outclassed by the P-38 and later fighters in the US inventory. The Soviets, on the other hand, loved the aircraft and used it as a ground attack aircraft.