The P-39 was developed originally to satisfy the US Army need for a dedicated ground attack aircraft. Hence the 37mm cannon which while limited in usefulness in a2a combat would theoretically be useful for ground attack. The resultant need to fit the engine behind the cockpit, in addition to making room for the huge cannon would further serve to protect the plane for ground straffing. The prototype was a nimble bird but by the time the production model was ready in 41 it was weighted down by the armor and other gear and resulted in it being underpowered. Since it was originally to be a ground attack aircraft it was not equipped with a supercharger which exaserbated it's performance issues above 15,000 feet.
P40 was a development/refinement/modification (depending on which org you ask) of an earlier fighter design and filled the role of general air fighter. A large contract was awarded to Curtis to fill out squadrons quickly. New, better aircraft were being developed of course but that was in the future.
Keep in mind that the US in particular was blessed with a multitude of aircraft companies, each privately owned and vying for government contracts so there was healthy competition. This fostered its shares of duplicate efforts, dead ends....and in the best case, some beauties that helped win the coming war.
Actually Nik the P-39 was desgined to fill the high altitude interceptor role. The 37mm was there to help destroy the bombers and other fighters. Remember that during the inter-war period .30 cal weapons were common and the .50 cals were not that common. The US didnt have a large calibre weapon until it was converted from the anti-tank gun of the same type mounted in the M3 light tank. The issue with it like a number of other auto-cannons from the inter-war and war was the low muzzle velocity and low rate of fire. The other guns were there to help defend the aircraft from the other fighters and help provide ranging fire to fire the 37mm cannon. In the prototype and experimental stage the P-39 and the Allison engine in it were fitted with a turbo-supercharger during this stage, also of note this is the same style of engine that was fitted to the YP-38A and P-51A; however, someone some place with in the USAAF portion of the War department asked for the engine to be simplified for production so the turbo-supercharger was deleted out of the all the major variants that were being ordered for the production version of the P-51, P-38 and P-39. In turn those aircraft were dogs until the turbo-supercharger was re-introduced (or in the case of the P-51 a licensed vesion of the Merlin was introduced into it).
The P-40 was a modification of the
Take my word for it. You never want to be involved in an “International Incident”.