An additional factor for the P-51 ( particularly the 'D' variant ) was that it was relatively easy to fly ; certainly easier than the Spitfire. Relatively novice pilots could get on with the business of fighting other aircraft without having to pay undue attention to actually flying - this was not the case with the Spitfire.

That's not my book-earned opinion ; it's a statement made to me by a veteran pilot who had 60 different aircraft types in his logbook......

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"Stand by to pull me out of the seat if I get hit" - Guy Gibson