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The Martin XB-51 was an unusual ground-attack plane, and one of the most highly advanced aircraft at the time of its first flight in 1949. Two jet engines were carried in pods near the nose and a third was buried aft under the tail. The pilot could vary the thin wing's angle of incidence in the air, making takeoffs and landings easier. The variable-incidence wing allowed a very long fuselage which carried two bomb bays, all fuel tanks, and the bicycle-style landing gear. Spoilers on each wing replaced conventional ailerons, allowing the use of full-span flaps for safer landings. The XB-51 was fast, maneuverable, and delightful to fly, but it lost an acquisition competition to the British-designed B-57 Canberra.
Development of the XB-51 was initiated in 1945, when the Army Air Forces (AAF) issued military characteristics for a light bomber aircraft. The AAF's requirements led to a design competition, held in February 1946. The Glenn L. Martin Company won the competition with a design for an airplane containing a composite power plant and promising a maximum speed of 505 miles per hour (438 knots), a cruise speed of 325 miles per hour (282 knots), and an 800 mile combat radius. The Martin design, then labeled the XA-45, also provided for a 6 man crew, all around armament, and high altitude bombing equipment.
The AAF military characteristics of 1945 were revised in the spring of 1946. The new requirements called for an aircraft with better performance for all weather, close support bombing. In line with Gen. H. H. Arnold's deletion of the requirement for "attack" aircraft--the revised characteristics also called for a redesignation of the Martin design, subsequently known as the XB-51.
Procurement of the experimental B-51 was initiated by a fixed price letter contract, issued on 23 May 1946. This agreement gave Martin $9.5 million to produce 2 XB-51s, to be preceded by the usual wind tunnel models and mockups. Special tools, spare parts, drawings, technical data, armament reports, and the like were also required.
The military characteristics of 1945 and 1946 were revised again in 1947 to satisfy officials of AAF Headquarters, who doubted that the XB-51, as then envisioned, would become a satisfactory light bomber. The possibility of seeking 1 or 2 new production sources was considered but given up after the Air Materiel Command pointed out that to stay with the XB-51 and use funds already obligated for this purpose was probably the surest way to acquire a light bomber that would not be obsolete before reaching the inventory.
Concurrent studies by Martin resulted in the design of an XB-51 aircraft with a top speed of 620 knots, a cruise speed of 463 knots, and a 378 mile radius of action. The revamped XB-51 was to be equipped with eight 20 millimeter cannon, be capable of carrying a 4,000 pound bomb load, and would require a 2 man crew, 4 men less than originally planned. Further design studies, conducted by Martin at the request of the Air Materiel Command, brought additional changes. More realistically, the revised XB-51's top speed was set at 521 knots and its cruising speed at 434. Since the XB-51 was intended essentially as a low altitude weapon, the radius requirement was decreased, bearing in mind that the Shoran (short range navigation) system earmarked for the plane was limited to less than 200 nautical miles. These final characteristics were approved by AAF Headquarters in early 1947. Shortly thereafter, the aircraft's development, in limbo for over a year, was re-instated.
Martin decided that a turbojet version of the basic XB-51 was the best configuration to satisfy the military characteristics that had been finally approved. Hence, the all metal, mid-wing monoplane was fitted with 3 J47 engines. Two of the 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.
The experimental XB-51 made its first flight on 28 October 1949. It was the Air Force's first high speed, jet propelled, ground support bomber, and was one of the first post war airplanes designed to destroy surface targets in close cooperation with Army ground forces.
Martin's letter contract of May 1946 was superseded on 1 November 1949 by a formal contract of the cost plus fixed fee type. This contract (W33-038-ac-14806), carrying the same number as the 3 year old letter contract, increased the amount initially obligated by $500,000 to cover the contractor's fixed fee.
Subsequently, change orders were to raise the cost of the ill fated, $10.2 million development contract. Meanwhile, the procurement requirements of 1946 remained unaltered. Martin was required to provide mockups, spare parts, technical data, and 2 XB-51s.
Testing of the first XB-51 was extensive. The Phase I tests, which lasted until the end of March 1951, indicated that relatively few modifications were needed and attested to the serviceability and excellent functional design of the experimental aircraft. Results of the Phase II tests, that had been conducted from 4 April to 10 November 1950, corroborated these findings. Martin pilots flew the first XB-51 (Serial No. 46-685) for 211 hours, accumulated in 233 flights. Air Force pilots totaled 221 hours on the same aircraft. The number of Air Force test flights was not accurately recorded, but did exceed 200. Flight testing of the second XB-51 (Serial No. 46-686), first flown on 17 April 1950, although thorough, was relatively brief. Martin test pilots flew the aircraft 125 hours, accumulated in 168 flights; the Air Force put in 26 hours, presumably reached in 25 flights. The second XB-51 was destroyed on 9 May 1952, during low level aerobatics over Edwards AFB, California. The pilot was killed as the aircraft exploded and burned upon striking the ground.
The Air Force accepted the 2 XB-51s built by Martin. The first one was accepted and delivered on 22 January 1952; (Delivery of the first XB-51 was delayed because of the extensive testing conducted by the contractor--a routine procedure.) the other during the previous month, on 8 December 1951.
The Air Force canceled production of the B-51 before the 2 experimental aircraft were formally accepted. Air Force records offered various reasons for the decision. For example, the XB-51 had received a second best rating in comparison with other aircraft designed to fulfill similar mission roles. Yet, these records failed to identify the aircraft which were compared and the factors that established the XB-51's disappointing rating. Considering the time invested in the XB-51's development (about 5 years), the Air Research and Development Command offered a more specific explanation. The command stated that termination of the XB-51 contract in November 1951 was due to the fact that the plane, in its existing configuration, did not meet the requirements, particularly the range requirement, of the Tactical Air Command.
Although Martin was informed in November 1951 that the XB-51 program was ended, the light bomber contract was not closed out until 7 October 1953, when a last change order was issued. This document had several important purposes. It instructed the contractor to repair the first of the 2 experimental aircraft which, though significantly damaged in February 1952, was the only remaining XB-51. The Air Force also instructed Martin to prepare the plane for bomb dropping tests and to send 2 field service representatives to participate in a 3 month bomb dropping program to be conducted at Edwards AFB. The final change order, in addition, determined the last sums owed to Martin. Included were $381,439 for the aircraft's repair, some $90,000 for the required special work and the field representatives' services, plus 2 fixed fees. Added to the expenses previously incurred for minor repairs and unexpected modifications, this brought the total cost of the experimental program to $12.6 million, a $2.4 million increase in about 4 years.
The Air Force did not determine the final disposition of the repaired and much improved XB-51. The aircraft was totally destroyed on 25 March 1956 in a crash at Biggs Field, Texas. In the meantime, however, a great deal was learned from the experimental program. The work performed by the 2 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.
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