In 1945, the US Army Air Forces issued a requirement for a light bomber aircraft. In February of 1946, a design competition was announced based on the USAAF requirements.
On April 1, 1946, the Glenn L. Martin Company of Baltimore, Maryland proposed a straight-winged, six-seat attack bomber powered by two TG-110 turboprops and two I-40 turbojets. The aircraft promised a maximum speed of 505 mph, a cruising speed of 325 mph, and a combat radius of 800 miles. The Martin design won the competition, and was assigned the designation XA-45 in the attack series.
In the spring of 1946, the USAAF revised its requirement, calling for an aircraft with better performance for all-weather, close-support bombing. The revised characteristics called for a redesignation of the Martin design as XB-51. A fixed-price letter contract issued on May 23, 1946 called for two XB-51s, to be accompanied by wind tunnel models and mockups.
The military characteristics specified in 1945 and 1946 were revised yet again in early 1947. The XB-51 was now pictured as a low-altitude attack aircraft and the combat radius requirement was reduced. The company designation of Model 234 was applied to the project.
The aircraft that finally emerged was powered by three General Electric J47 turbojet engines, one in the tail fed by a top air inlet and two in nacelles underneath the forward fuselage. The wings were swept back at 35 degrees and had six degrees negative dihedral. The wings had variable incidence to enhance performance for takeoff and landing The wings were fairly advanced for the day, having spoilers instead of ailerons and sporting leading-edge slots and full-span flaps. The crew was two, consisting of a pilot seated underneath a bubble type canopy and a navigator seated behind him within the fuselage. The landing gear was similar to that of the B-47--consisting of a set of tandem dual mainwheels which retracted into the fuselage and supported by a set of small outrigger wheels which retracted into the wingtips. An unusual feature was the use of a rotatable bomb bay door on which the bombs were mounted. When open, the weapons bay load was essentially the same as with external stores, but without the speed restrictions.
The XB-51 prototype (46-0685) flew for the first time on October 28, 1949. It was the USAF's first high-speed, jet-powered ground support bomber.
Phase I tests, which lasted until the end of March 1951, indicated that the design required relatively few modifications. Phase II tests, carried out between April and November 1950 confirmed these findings. Martin test pilots flew the XB-51 for 211 hours in 233 flights. Air Force pilots carried out 221 hours of test flights.
The second XB-51 (46-0686) flew for the first time on April 17, 1950. It was fitted with an armament of eight 20-mm cannon in the nose, with 160 rpg. Up to 10,400 pounds of bombs could be carried, but the basic mission consisted of the delivery of 4000 pounds over a 475-mile radius.
In 1950, following the beginning of the Korean War, the USAF perceived a need for a night intruder bomber to replace the Douglas A-26 Invader. The XB-51 was entered in the contest, along with the North American B-45 Tornado and the North American AJ-1 Savage. Foreign entries included the Avro Canada CF-100, a twin-jet all-weather interceptor, and the English Electric Canberra. On December 15, 1950 a Senior Board of officers recommended that the XB-51 and the Canberra had the best potential as a night intruder. Although a relatively large aircraft, the XB-51 was highly maneuverable for its size. At low levels, it had a very satisfactory turning radius in the speed range of 280-310 IAS. However, its low limit load factor of 3.67 G severely limited its capability during tactical operations, and was generally considered unsatisfactory. The XB-51 was nearly a hundred knots faster than the Canberra at low level, its maximum speed of Mach 0.89 below 30,000 feet made interceptions of the XB-51 by aircraft such as the F-86 extremely difficult. However, the endurance of the XB-51 was much poorer than that of the Canberra, with the Canberra being able to loiter for 2 1/2 hours over a target 780 nautical miles from its base. The XB-51 could loiter only one hour over a target 350 nautical miles from its base. Despite the prospect that improved jet engines would eventually be available, there was little prospect that the range and endurance of the XB-51 would improve sufficiently to meet the loiter time requirement. In addition, it was thought that the small outrigger wheels on the XB-51 might be troublesome at hastily-prepared forward air bases. In early 1951, a flyoff at Andrews AFB finally settled the issue, and the Canberra was declared the winner. On March 23, 1951, 250 examples of the Canberra were ordered under the designation B-57A.
The XB-51 program was cancelled in November of 1951. However, Martin was not all that upset, since they were awarded the contract to build the B-57.
Flight tests with both prototypes continued after program cancellation. The second XB-51 (46-686) crashed on May 9, 1952 during low-level aerobatics over Edwards AFB, killing its pilot. The first prototype XB-51 (46-685) continued on with various other test work. Extensive tests on high-speed bomb release were carried out, and the tail configuration, variable incidence wing, and bicycle-type landing gear provided much useful data. The XB-51 even starred in a movie--the film "Toward the Unknown" starring William Holden, Lloyd Nolan, and James Garner in which it was assigned the spurious designation "Gilbert XF-120". The aircraft was totally destroyed on March 25, 1956 when it crashed on takeoff from El Paso International Airport. It rose to about 400 feet or so, and suddenly nosed down and went straight into the desert just off the end of the runway. Both crewmembers were killed.
Engines: Three General Electric J47-GE-13 turbojets, each rated at 5200 lb.s.t. Performance: Maximum speed 645 mph at sea level. Cruising speed 532 mph, landing speed 153 mph. Service ceiling 40,500 feet. Initial climb rate 6980 feet per minute. Normal range 1075 miles, maximum range 1613 miles. Weights: 29,584 pounds empty, 55,923 pounds gross, 62,457 pounds maximum. Dimensions: wingspan 53 feet 1 inches, length 85 feet 1 inches, height 17 feet 4 inches, wing area 548 square feet. Armament: Eight 20-mm cannon with total ammunition capacity of 1280 rounds. Normal bombload was four internal bombs of 1600 lb. each or two external bombs of 2000 pounds each. Maximum bombload of 10,400 pounds.