And so continues another myth - the turretless P-61s lost out to the greater needs of the B-29 (this in the Osprey page included above). Instead, the truth is that the P-61 turrets weren't working for a variety of reasons. Rather than install them, they were put on the shelf waiting for an effective fix. The B-29 folks noticed the "extra" turrets lying around and grabbed them - of course, there were mods necessary to the B-29 and the turret in order to marry them together...
Cheers,
Dana