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The Boeing XB-55 program was essentially an enlarged version of the basic B-47 design with turboprop engines rather than jets. Boeing planned to use four Allison T40-A2 turbines each driving a dual-contrarotating propeller. These engines gave the design an estimated top speed of 490 mph; however, the performance of the B-47 was far better. Since even better all-jet bombers were being designed, the XB-55 program was canceled while still in the design phase.
The turboprop engine was eventually tested on a USAF bomber when two B-47s were modified for Wright YT49 turboprops. The aircraft, designated XB-47D, were used to test the feasibility of using turboprops for bombers. This project was unsuccessful also, but did provide some valuable test data on the high speed performance of turboprop engines.
TECHNICAL NOTES (as designed):
Armament: 12 20mm cannons and 24,000 lbs. of bombs
Engines: Four Allison T40-A2 of 5,600 lbs. hp each
Maximum speed: 490 mph
Cruising speed: 450 mph
Range: 5,000 miles
Service ceiling: 42,000 ft.
Span: 135 ft. 0 in.
Length: 118 ft. 11 in.
Height: 33 ft. 8 in.
Weight: 168,000 lbs. (estimated max. takeoff weight)
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