The XF-81 was designed to solve the very limited range problem of early jet fighters. The aircraft had two engines: a General Electric XT-31 gas turbine turboprop of 1,650 hp and an Allison J-33 turbojet of 3,750 lbs. thrust. The concept was to use the turbojet only during combat and takeoff and the turboprop for cruise.
Two XF-81s were ordered (S/N 44-91000, 44-91001) on Feb. 11, 1944. The first flight was Feb. 11, 1945; however, because of delays with the turboprop engine, a Packard V-1650 Merlin engine was fitted to the aircraft instead. Convair retrofitted the first aircraft for the XT-31, which flew for the first time with the design configuration on Dec. 21, 1945.
Performance with the turboprop was barely better than with the Merlin and disappointing overall. An order for 13 YF-81s was canceled, but the XF-81s continued as test aircraft until Sept. 29, 1947, when they were declared obsolete.
First USAF turboprop fighter
TECHNICAL NOTES: Armament: Designed for six .50-cal. machine guns or six 20mm cannon and two 1,000-lb. bombs Engines: One Allison J-33 and one General Electric T-31 gas turbine Maximum speed: 507 mph Cruising speed: 275 mph (turboprop only) Range: 2,500 mi. Service ceiling: 35,500 ft. Span: 50 ft. 6 in. Length: 44 ft. 10 in. Height: 14 ft. 0 in. Weight: 24,650 lbs. maximum Crew: One