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:
P-39 was really good at low altitudes, but became slow and clumsy at high altitudes, so it was shit in the Pacific, where most air fights were at high altitudes, but excelled at the Eastern front, where most air fights happened at low and medium altitudes. And also it had shit ton of firepower - 1 37-mm gun and 2-4 12,7-mm machineguns.
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.