There is an article about P-61s here:
https://www.airspacemag.com/history-...ows-180959772/
It points out that the top remote turrets were in short supply (the same turret was used on B-29s) and about half of the P-61s built just had 4 fixed forward firing guns in place of the turret. Some pilots would still take a gunner along as an extra pair of eyes, but it appears that most did not.