With their innovative XA-45 attack aircraft, Martin was building on the success of their B-26 Marauder medium bomber. Conceived in 1945 as Martin’s answer to a US Army Air Force (USAAF) requirement for a low-level bomber, the aircraft featured a number of ground-breaking design characteristics. When the war ended and the USAAF did away with the A prefix for attack aircraft, the jet became the XB-51. Due to its unique design the aircraft graced many a magazine cover during the 1950s and even starred in a movie. Nice work if you can get it. But when the chips went down things cooled in a hurry.
But I digress. At first the XA-45 was designed with straight wings and was to be powered by a hybrid twin turboprop/twin turbojet installation. When the USAAF radically revised the requirements for the XA-45 in the spring of 1946, the aircraft’s mission changed to all-weather close-support bombing. A higher performance aircraft was needed. At this point the aircraft was redesignated as XB-51. The Air Force issued a contract for two XB-51s on 23 May 1946. As might be expected, their minds changed again in 1947- this time seeing the XB-51 as a low-altitude attack aircraft with a reduced combat radius. Once Martin assigned the project a model number (Model 234 in the case of the XB-51) things (finally) began to move forward.
The Model 234 design featured variable-incidence wings swept at 35 degrees and a pronounced 6 degree anhedral. Full-span leading edge slats and trailing edge slotted flaps increased lift resulting in shorter takeoffs. Spoilers and small ailerons were used for roll control. The empennage was configured as a swept T tail. The most unique feature of the design had to be the choice of three General Electric J47-GE-13 axial-flow turbojet engines- two housed in low-mounted pods on either side of the forward fuselage and the third in the tail of the aircraft breathing via an intake located at the base of the vertical stabilizer.
Additional thrust for takeoff was supplied by four rocket assisted take off (RATO) bottles. The main landing gear wheels of the XB-51 were configured in tandem under the forward and mid fuselage with outrigger wheels mounted at the wingtips- similar to the Boeing B-47 and B-52. This configuration was tested by Martin on a modified B-26 nicknamed Middle River Stump Jumper. Other design characteristics of the jet were the near-nose bubble canopy under which the pilot sat. The second crew member, a bombardier, was seated inside the pressurized and climate-controlled fuselage below and behind the pilot.
Both crew members were provided with upward firing Martin-designed ejection seats- a first for Martin aircraft. Other innovative features of the XB-51 were its rotary bomb bay (trademarked by Martin) and an external load capacity of 10,400 pounds. Also designed into the airframe were provisions for eight nose-mounted 20 millimeter cannons. Performance of the jet was impressive, with a cruise speed of 532 miles per hour at altitude and a maximum speed of 645 miles per hour at sea level. The service ceiling was 40,500 feet and range was 1,075 miles- all exceeding the design requirements (which remember changed several times during development) by a fair margin.
The XB-51 measured more than 85 feet in length with a wingspan of slightly more than 53 feet. Pictures of the jet from above or below highlight the length of the aircraft- perhaps contributing to the less than flattering nickname of “Flying Cigar.” The T-tail stood more than 17 feet off the ground. But weighing in at more than 57,000 pounds when loaded and still being capable of speeds just under the speed of sound made the aircraft an impressive specimen and garnered interest in the jet not just within the Air Force but around the world.
The first XB-51 prototype (Air Force serial number 46-0685) was rolled out of the Martin factory near Baltimore on 4 September 1949 and flew for the first time on 28 October 1949. Initial testing concluded at the end of March 1951 yielding few required modifications. Flight testing by Martin totaled 211 hours over 233 flights. United States Air Force (USAF) testing totaled 221 hours. When the second XB-51 (serial 46-0686) took to the skies for the first time on 17 April 1950 it was armed with those eight 20 millimeter cannons in the nose and later spent time at Edwards AFB flown by several Air Force pilots including none other than Major Charles E. “Chuck” Yeager. But it wasn’t long before Korea forced a separation between testing and practical application.
The Air Force determined a need for a B-26 Invader replacement in the night intruder role. The North American B-45 Tornado, the North American AJ-1 Savage, the Avro Canada CF-100 Canuck, the English Electric Canberra, and the XB-51 were considered. The CF-100 was eliminated early on and the Tornado and Savage had other missions, but the Canberra and the XB-51 fought it out. The Canberra was a fine machine and a capable bomber. The XB-51 could easily outpace it. In fact the XB-51 could outperform many of the jet fighters of its day- as long as it wasn’t asked to turn with them due to a low load-limiting factor of only 3.67 Gs. The XB-51’s landing gear was not suitable for austere forward airfields either. Loiter time over target pretty much settled it- the Canberra had it; the XB-51 didn’t.
The Air Force chose the Canberra as their new medium bomber. But the Glenn L. Martin aircraft company was chosen to build Canberras under license on 23 March 1951. The Martin B-57A Canberras built for the USAF were equipped with the rotary bomb bay designed into the XB-51. The unique XB-51 prototypes continued to fly research flights even after the type’s cancellation. But 46-0686 crashed at Edwards AFB while performing low-level aerobatics on 9 May 1952, leaving 46-0685 as the sole XB-51. The aircraft’s unique appearance no doubt helped it land the part of the Gilbert XF-120 in the 1956 Warner Brothers film “Toward the Unknown.” But after 46-0685 crashed on takeoff at El Paso International airport on 25 March 1956 while en route to Eglin Air Force Base (AFB) to shoot footage for the movie, the XB-51s were no more.
Speculation about the reasons for the decision to build the B-57A instead of the B-51 ranges from head scratching to passionate debate about the wondrous performance of the XB-51 and how great it would have been while equipping 15 or 20 Air Force bomb squadrons. In the end the Air Force probably made the right decision. The B-51 would not have been as adaptable and flexible as the B-57 turned out to be- it had no growth potential. The XB-51s had speed to burn but were not very maneuverable and their range and loiter time were inferior to the Canberra. The bottom line is we’ll never know of what else the XB-51 was capable. They were radically advanced aircraft at a time when aerospace engineering was advancing just as fast as they were capable of flying. But they sure were cool!