When the Martin XB-51 made its initial flight on 28 October 1949; it became the USAF’s first high speed, jet propelled, ground support bomber, and was one of the first post war airplanes that were designed to destroy surface targets to support the ground forces.

The Martin XB-51 was one of the most highly advanced aircraft at its time. It was fast, maneuverable, and delightful to fly; but it lost momentum as soon as the British-designed B-57 Canberra came in the picture. 

Development of the aircraft started in 1945, when the Army Air Forces issued characteristics for a light bomber aircraft. The Glenn L. Martin Company managed to convince the AAF with its design. Martin suggested that the aircraft will have a composite power plant and promising a maximum speed of 505 miles per hour, a cruise speed of 325 miles per hour, and an 800 mile combat radius. The Martin design also provided for a 6 man crew, all around armament, and high altitude bombing equipment. It was than, known as XA-45. 

In the spring of 1946, AAF revised its characteristics. The new requirements asked for better performance on all weather and close support bombing. Gen. H. H. Arnold’s ambition of ‘attack’ aircraft called for a re-designation of the Martin design. Martin did that and named it as the XB-51.

In XB-51, two jet engines were incorporated in the pods near the nose and a third was buried aft under the tail. The pilot could change the thin wing's angle of incidence in the air, during takeoffs and landings. Its ‘variable-incidence’ wing allowed a long fuselage which carried two bomb bays, all fuel tanks, and the landing gear. Spoilers on each wing allowed the use of full-span flaps for safer landings. 

On 23 May, 1946, The AAF issued the contract to purchase the experimental B-51. This agreement gave Martin $9.5 million fund to build two XB-51s. Again in 1947, the AAF revised its design to satisfy the top officials who doubted that the aircraft would become a satisfactory light bomber. The AAF considered 1 or 2 new production sources but later gummed with the XB-51.  Martin came up with new design that had a top speed of 620 knots, a cruise speed of 463 knots, and a 378 mile radius of action. The new XB-51 was equipped with eight 20 millimeter cannon, be capable of carrying a 4,000 pound bomb load, and would require a two man crew, four less than originally planned. 

Further design studies by Martin at the request of the Air Materiel Command, brought additional changes. The revised XB-51's top speed was set at 521 knots and its cruising speed at 434. The radius requirement was decreased since the aircraft was intended essentially as a low altitude weapon. The AAF approved these characteristics in early 1947, making way for the development of XB-51. During the manufacturing stage, Martin decided to install a turbojet version to satisfy the military needs; so it fitted J47 engines in the aircrafts. Two of these engines were in nacelles mounted on pylons on the lower forward sides of the fuselage, while the third engine was carried internally in the rear fuselage, with a top air inlet and a jet exit in the aircraft's tail.

First flight was tested in October 1949. The first phase tests indicated the serviceability and excellent functional design of the experimental aircraft. Results of the Phase II tests, done from 4 April to 10 November 1950, also confirmed these findings. Martin pilots flew the first XB-51 for 211 hours, accumulated in 233 flights. Air Force pilots totaled 221 hours on the same aircraft.

Martin test pilots flew the second XB-51, which was first flown on 17 April 1950, for 125 hours, accumulated in 168 flights. The Air Force put in 26 hours, presumably reached in 25 flights. On 9 May 1952, a tragic accident during a low level aerobatics over Edwards AFB, California destroyed the second XB-51. In November 1951, the Air Force canceled production of the B-51 before the two experimental aircrafts were formally accepted. The Air Research and Development Command said that cancellation of the contract was due to the fact that the plane, in its existing configuration, did not meet the requirements, particularly the range requirement.

On 7 October 1953, the Air Force instructed Martin to repair one of the experimental aircraft which was damaged in February 1952. The Air Force also instructed Martin to prepare the plane for bomb dropping tests and to send two field service representatives to participate in a three month bomb dropping program to be conducted at Edwards AFB. The order, in addition, determined the last calculations; including $381,439 for the aircraft's repair, $90,000 for the required special work and the field representatives' services, plus the two fixed fees. These additional fees brought the total cost of the experimental program to $12.6 million, a $2.4 million increase then the initial accounts.

The work performed by the two XB-51s in the high speed bomb release program contributed much to advancing the state of the art in that field. Also, the tail configuration, variable incidence wing, and bicycle type landing gear of the XB-51 provided useful design data.

The last serving XB-51 was destroyed on 25 March 1956 when it crashed at Biggs Field, Texas.