The first P-39C (Ser No 40-2971) flew in January of 1941. The Army discovered almost immediately that the P-39C was not combat ready, since it lacked armor and self-sealing tanks.
Gee, your much toted P-39 did not have armor and self-sealing fuel tanks and flew some 17 months after WW2 had started.
One would think that lessons would have been learned by then and been incorperated in the design from the start.