Hmm...as far as I know, the 'D' was an Experimental model, the XP-61D. The aircraft was an attempt to improve the Widow's high-altitude performance. The first XP-61D was a P-61A-5 s/n# 42-5559; the P&W R-2800-65 engines were replaced by P&W R-2800-77 w/GE CH-5 Turbo-Superchargers. A second a/c was converted from a P-61A-10 (42-5587). Both a/c flew, and High-Altitude performance was naturally improved, this being the point of supercharging, after all. The XP-61D suffered numerous engine failures (a fault of several US high-alt, high-output engines), and the a/c was superceded by the P-61C. There were no production model 'D' a/c. 07/31/2004 @ 19:57 [ref: 7949]