As you may recall, work on the XP-61C had initially proceeded quite slowly at Northrop because of the greater priority of the XB-35 flying wing bomber. Consequently, most of the work on the XP-61C was contracted out to Goodyear Aircraft of Akron, Ohio. Goodyear was a natural choice for this work, since the company was already doing some subassembly work for Northrop on the Black Widow.
It was initially planned that Pratt & Whitney R-2800-77 engines with General Electric CH-5 turbosuperchargers were to power the XP-61C. However, because of the demands of higher-priority projects for this engine, the R-2800-14W was substituted. In reality, there were few differences between these engines. In mid-Feruary 1944, arrangements were made to divert a P-61A-5-NO (serial number 42-5559) to the XP-61C project. As a safety measure, Northrop suggested that a second Black Widow be converted as well, and since the P-61A production line was on Block 10 at that time, a P-61A-10-NO (serial number 42-5587) was taken off the production line for conversion to XP-61C standards.
By late April 1944, the engines had been changed yet again, to the R-2800-57. On April 27, Wright Field ordered that the designations of these two experimental aircraft be changed to XP-61D so that they would not be confused with the production C models. The XP-61D was to be modified to carry four wing pylons (two inboard and two outboard of the engine booms)
Technical difficulties and labor problems at Goodyear delayed the XP-61D all throughout mid-1944, and it was not until November 1944 that the first flight took place. By this time, the engines were a pair of R-2800-77 turbosupercharged radials, rated at a war emergency power of 2800 hp. The flight test program for the two XP-61Ds was completed by the fall of 1945. Maximum speed was 430 mph at 30,000 feet, and an altitude of 30,000 feet could be attained in 13.5 minutes. Service ceiling was 43,000 feet. Weights were 23,305 pounds empty, 29,850 pounds gross, and 39,715 pounds maximum.
By this time, the basically similar P-61C had already entered production, and consequently the XP-61D had a very low priority. The end of the war in the Pacific caused the priority of the XP-61D project to drop to zero. The first XP-61D was scrapped on September 11, 1945, and the second one in April 1946.