P-40 Warhawk Allison vs. Merlin

The Curtis P-40 was built with both the Allison V-1710 and the Packard Merlin V-1650. Why did they build it with both engines? Which was better?

I'll cover both engines as used in various models including the E, K, F, L, M, and N. The F and L models are Merlin powered.

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