It was developed as an improved version of the P-39. The fuselage design is aerodynamically sleek, it had a better roll rate than the P-47 and P-51, it reached speeds of over 400 mph, like all American fighters it was rugged. The Soviets had success with it. I understand it was developed during in the war, and at that point the P-51 was considered superior and more favored by the AAF, but it seemed to have solid qualities, and with success in the Soviet front against the Germans, why not equip some squadrons with them to add depth to their materiel. They also made a swept-wing version of this plane.
The Sovs had success because they used it at lower levels, where they fought their war. The P-63 wouldn't have survived in the high altitude Western European air war. There was no logic for the Army to field too many different designs, which is why proven all around designs were maintained.
To expand on what Gromit has already posted, The US produced a considerable variety of fighter aircraft for their own service and another that did not offer enough difference in performance was of limited use and only complicated logistics. A P-47 was extremely rugged, A P-51 was a nimble and long ranged, high altitude escort, a P-38 offered the security of a second engine for long over water flights as well as range. Add the P-40 which flew in US service till late 1944 in various roles. The Navy had three designs they favored F6F, F4U, F4F (and FM variants) that filled their needs, so having a design in the P-39/P-63 that Russians actually liked and wanted meant you could fulfill Lend-Lease without compromising US needs overly.
They could have kept it on the backburner in reserve for the USAAF in case there were heavy losses while they were shipping them to Russia, because they seemed to have been a marked improvement from the P-63, maybe plugging them into the P-40 roles as fighter/bomber in the secondary role. That's probably a moot point though.
It was an odd bird...Never understood the car door access. How could a pilot abandon ship if there was a slipstream that prevented pilot from opening door to eject?...maybe there was an emergency hatch
Ahh. Thanks... Would worry about the left tail after jumping...lots of stories of pilots getting hung up.
If you roll out, you are already under the horizontal stabilizer. Of course, there are a lot of other factors for a successful bailout.
It seemed like a plane that could have performed on par with the other main-line fighters, good rate of climb, speed, and other attributes are solid.
The breakdown was roughly... route, Amount shipped, Arrived/lost North Russia, 3,964,000 tons, 93%/7% Persian Gulf, 4,160,000 tons, 96%/4% Black Sea, 681,000 tons, 99%/1% Far East, 8,244,000 tons, 99%/1% Soviet Artic, 452,000 tons 100%/0% Various breakdowns of L-L shipments to Russia can be found here: http://www.o5m6.de/Routes.html