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The Northrop XP-61D was the second designation given to the two prototypes for the up-engined P-61C Black Widow, and was adopted after a series of engine changes. It was originally intended to use turbo-supercharged Pratt & Whitney R-2800-77 engines on the P-61C. A shortage of these engines meant that Goodyear, the company given responsibility for the project, had to use Naval R-2800-14W engines. These were then replaced by R-2800-57s, and at this point the two aircraft were redesignated as XP-61Ds. The modified aircraft lived up to expectations, and was ordered into production as the P-61C, although the end of the war meant that only 54 were completed before production ended.