The early strength of American fighters was their ability to absorb hits from opposing fighters. When armed with the 50 cal machine guns, they were also more lethal because of the amount of firepower they could muster on a target.
I left the P-36 out of the mix because it was mostly not a factor. Then there was the Brewster Buffalo...
BTW the Russians loved the P39 as a low altitude ground support role. But in the air superiority role it stunk on ice.