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The Martin XB-68 is, in my opinion, one of the best looking stillborn aircraft projects of the 1950s. Development of the XB-68 began in 1954, in response to a USAF request for a supersonic tactical bomber to satisfy the WS-302A requirement. A top speed in excess of Mach 2 was envisioned, with the aircraft conducting the bombing run at high altitude. Three companies, Martin, Douglas, and North American, submitted designs. Martin's design was assigned the internal company number 316. Initial concepts off all three entries clearly show aircraft optimized for high speed flight; Martin; Douglas: North American: (Douglas and North American images via source (1)) (I am not certain if there is any connection between the design of the Vigilante and North American's WS-302A entry. Given the similar operational requirements and the timeframe, it is highly likely the A3J was as least somewhat based on the WS-302A design). As can be seen, the original design of the XB-68 had roughly 45 degrees of wing sweep. Additionally, it was to have used the J67 turbojet engine, an American version of the Rolls Royce Olympus. (The J67 was also considered for several other aircraft, including the XF-103). Both of these features would change as the XB-68 evolved. At this time, the aircraft had a maximum weight of about 96,000 pounds, and 900 square feet of wing area. Two crewmembers sat in tandem in a highly streamlined cockpit. The Martin 316 was selected as the winner of the competition in 1956, and selected for further development. At the time, plans were for the aircraft to enter service in the early 1960s. As the design underwent further development, several changes were made. The J67 engines were discarded, and replaced with J75 turbojets. The exact reasoning behind this was unknown, but the J67 (built by Curtiss-Wright) had a very troubled development and was ultimately stillborn. In contrast, Pratt & Whitney's J75 was successfully used in several other aircraft, including Martin's P6M that was also being developed in the 1950s. (The dihedral on the stabilizers of both the P6M and XB-68 shows common Martin influence). The second change was moving to a less swept, trapezoidal wing, rather than the swept wing of the initial design. Studies in the NASA Langley wind tunnel (4) showed that a wing swept at only 19.2 degrees had less drag at high mach numbers than the 45 degree sweep wing, due to the thinner airfoil that could be used. The result was an aircraft that resembled another 1950s aircraft intended to operate at Mach 2 at high altitudes; the F-104 Starfighter. The design of the XB-68 was finalized by approximately 1957. The final aircraft had a maximum takeoff weight of 100,000 pounds, with a wing area of 875 square feet. (3). With a length of 109.8 feet, but a wingspan of only 53 feet, it was more than twice as long as it was wide. Using the two J75s, top speed at the maximum altitude of 57,250 was to have been 1357 knots, roughly Mach 2.35. However, when fitted with a water injection system, the XB-68 could have reached altitudes in excess of 60,000 feet, and a slightly higher top speed. According to (3), this limit was imposed by the structure of the aircraft (likely aerodynamic heating). The primary armament of the XB-68 was never fully decided on; according to (3), up to 8,500 of bombs would have been carried in a rotary bomb bay (this was the payload over a short range, at maximum range the payload would have been limited to about 4,000 pounds). Given the time and the aircraft's intended mission, munitions could have included the Mark 7, Mark 10, and Mark 28 nuclear weapons. The XB-68 was also to have been equipped with a 20mm T171E2 rotary cannon mounted at the extreme rear of the aircraft. The XB-68 made it as far as mockup form, before being cancelled in the late 1950s; With how ambitious the XB-68 was, the aircraft would likely have not entered service until the mid-1960s. A development time of over a decade, while short today, was a relative eternity in the 1950s. Another problem was the development of surface to air missiles such as the S-75. The development of these missiles, and the Soviet deployment of them in Europe would have placed the XB-68 in severe danger. Granted, the XB-68 would have performed well at low level. Source (3) indicates it was projected to achieve Mach 1.25 at sea level, and the high wing loading (114 lb/ft2, vs 148 lb/ft2 for the F-104) would have given it excellent stability at low level. While the similar looking F-104 suffered severe accident rates in the low level strike role, the XB-68 would certainly not have been given to green West German crews. However, the F-104 was much cheaper, and had an air to air capability which the XB-68 lacked. However, it is possible that the XB-68 could have found a niche as a low level penetration bomber with superior range to the F-104. In this role it would have been soon eclipsed by the F-111, which first flew in 1964, around when the XB-68 would have entered service. Sources: (1)https://www.secretprojects.co.uk/threads/usaf-weapon-system-302a-tactical-bomber-competition.22864/ (2) https://www.secretprojects.co.uk/threads/martin-model-316-xb-68-tactical-bomber.479/ (3) http://www.up-ship.com/apr/v0n0.pdf (4) https://digital.library.unt.edu/ark:/67531/metadc64207/m1/2/