The P-39 was HATED by the Americans so they offloaded pretty much all of them to the Soviets. The Soviets tore up the sky with the P-39, producing many Soviet aces.
Because of these Soviet pilots, the Bell P-39 Airacobra has the highest number of kills attributed to any U.S. fighter type flown by any air force in any conflict.
:stalin-approval:
It's basically the opposite of the Spitfire, which was a high altitude fighter and lend lease Spitfires performed terribly in the low altitudes fights in the Eastern Front.