If you want to know if XP-55 could have been an alternative to the P-51: the answer is no. The XP-55 in a mock up combat is an easy prey for the P-51. That's because the XP-55 was not designed for the air superiority role as the P-51. The XP-55 was designed as a bomber interceptor, not for fighter to fighter combat.
XP-55 was pretty much a lemon with a poor engine and some nasty attributes such as to yaw with little or no warning. It was also prone to stall easily and when one was lost during stall testing, it stalled quickly, rolled inverted and fell out of the sky like an anvil.
If you can get a copy second hand (I think its out of print) USAAF fighters of WW2 by Michael O'Leary has good write ups on the "main types" as well as the XP-55, XP-46, XP-56, XP-67 (lovely looking bird) and others.