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.