On the Eastern Front, the strategic surprise of Operation Barbarossa demonstrated that Soviet air defense preparations were woefully inadequate, and the Great Purge rendered any lessons learned by the Red Air Force command from previous experience in Spain and Finland virtually useless. During the first few months of the invasion, Axis air forces were able to destroy large numbers of Red Air Force aircraft on the ground and in one-sided dogfights. However, by the winter of 1941–1942, the Red Air Force was able to put together a cohesive air defense of Moscow, successfully interdict attacks on Leningrad, and begin production of new aircraft types in the relocated semi-built factories in the Urals, Siberia, Central Asia and the Caucasus. These facilities produced more advanced monoplane fighters, such as the Yak-1, Yak-3, LaGG-3, and MiG-3, to wrest air superiority from the Luftwaffe. However, Soviet aircrew training was hasty in comparison to that provided to the Luftwaffe, so Soviet pilot losses continued to be disproportionate until a growing number of survivors were matched to more effective machines. Beginning in 1942, significant numbers of British, and later U.S., fighter aircraft were also supplied to aid the Soviet war effort, with the Bell P-39 Airacobra proving particularly effective in the lower-altitude combat typical of the Eastern Front. Also from that time, the Eastern Front became the largest arena of fighter aircraft use in the world; fighters were used in all of the roles typical of the period, including close air support, interdiction, escort and interception roles. Some aircraft were armed with weapons as large as 45 mm cannon (particularly for attacking enemy armored vehicles), and the Germans began installing additional smaller cannons in under-wing pods to assist with ground-attack missions. -Cmd. Sgt. Maj. McKee
I think I remember reading that most of our guy's were not too impressed with the P-39 but the Russian pilots loved it. It turned into a kind of 'Hand-Me-Down' aircraft as we transitioned to the P-47 and P-51.