The P-39 Airacobra was a unique fighter aircraft with its engine mounted behind the pilot to make room for a 37mm cannon that fired through the center of the propeller hub. This made it an effective fighter at low to medium altitudes. But its lack of supercharger or turbocharger penalized its performance at high altitudes. Many P-39s were given to the Soviet Union under Lend-Lease, and they made great use of it as a fighter as the air war on the Eastern Front was generally a lower-level affair.