[Alemart_el_Redentor]

... It was cancelled after six planes were finished, because the P-51D and P-47N were available and capable already for what was needed at the time...

I read somewhere that the fact that the allies used the nuclear bombs successfully (and thus, there was no need to invade mainland Japan) was the determinative factor for the cancellation, also that it was never actually intended to be used, but rather to keep certain companies "busy".