The B28, originally Mark 28, was a thermonuclear bomb carried by U.S. tactical fighter bombers, attack aircraft and bomber aircraft. From 1962 to 1972 under the NATO nuclear weapons sharing program, American B28s also equipped six Europe-based Canadian CF-104 squadrons known as the RCAF Nuclear Strike Force. It was also supplied for delivery by UK-based Royal Air Force Valiant and Canberra aircraft assigned to NATO under the command of SACEUR. In addition, certain U.S. Navy carrier based attack aircraft such as the A3D (later A-3B) Skywarrior, A4D (later A-4) Skyhawk, and A3J (later A-5A) Vigilante were equipped to carry the B28. Production history Initial development of the IN and EX weapons During the design of the TX-15 in 1953 it became evident to designers that massive reductions in size and weight of thermonuclear weapons were possible. In November 1954 the TX-Theta Committee proposed the development of the WX-27 and the WX-28. The larger diameter TX-27 was intended as a missile warhead or for internal carriage in aircraft while the smaller diameter TX-28 would be internally or externally carried on high-performance bombers. In a meeting in December the possibility of a small warhead allowing for smaller missiles was considered so the use of the TX-28 as a warhead was also included. Design of the XW-28 was given to Los Alamos National Laboratory with Sandia National Laboratories working on the non-nuclear components. In February 1955 Sandia proposed that a basic warhead assembly be designed and that different noses, afterbodies, fins and fuzes could be attached to it. Emphasis was placed on obtaining the optimum yield possible within the space and weight limitations assigned which necessitated having a much thinner case design than in previous weapons. Sandia presented a design outline of the weapon to the Division of Military Application in May 1955. It was decided that if the weapon could not be fitted to all of the required aircraft, first priority would be given to developing a bomb designed for subsonic internal or external carriage. Designers hoped though that they could produce a weapon considerably smaller than the and diameter specified. In April 1955 Sandia were working on the fuze design. Barometric fuzing was rejected due to the wide variety of delivery systems the weapon would use. A timer was also considered having benefits in low-level delivery missions, but it was felt that these gains were slight and did not justify the