The Brits wanted more P-40s than Curtiss could supply, and thought NAA would welcome the business.
NAA looked over the Curtiss specs and said in effect, "We can do better." And they did.
Having heard this DB 605 at full chat close up...there is no way to reproduce this sound. It's more than a sound, it's literally a visceral experience.
In the view of this once-upon-a-time aero engineer, the P-39 was the victim of its layout, combined with its diminutive size. The big gun and nosewheel filled the space where you would find the engine and a fuel tank. The water and oil coolers occupied the space where fuel might have been...