B28

The B28, originally designated Mark 28, was a thermonuclear weapon developed in the mid-1950s. Its origins trace to November 1954, when the TX-Theta Committee proposed the WX-28 as a thermonuclear weapon for internal or external carriage on high-performance bombers or as a missile warhead. Design work began in 1955, focusing on maximizing yield within space and weight limitations. The initial external and internal configurations were design-released in June 1957, with production starting in August 1958.

The B28 utilized a modular design allowing various combinations of noses, afterbodies, fins, fuzes, and warheads. It was produced in five primary configurations: the B28EX (streamlined external free-fall), B28IN (unstreamlined internal free-fall), B28RE (streamlined external parachute-retarded), B28RI (unstreamlined internal parachute-retarded laydown), and B28FI (unstreamlined internal full-fuzing laydown). Fuzing options included a combination of radar and contact fuzing, with ground-selectable contact preclusion to disable contact detonation during airburst missions. Later retarded and laydown variants incorporated velocity-sensing arming systems, acceleration-integration safety systems to prevent arming if parachutes failed, and crushable honeycomb nose structures to survive low-altitude ground impacts.

The weapon was produced in multiple modifications, from Mod 0 through Mod 4. Mod 0 used internal initiation, while subsequent modifications utilized external neutron initiators. Mods 3 and 4 incorporated Permissive Action Links. Yield configurations spanned five variants: Y1, Y2, Y3, Y4, and Y5. A derivative warhead, the W49, was adapted for Atlas, Thor, Jupiter, and Titan I ballistic missiles by removing the internal power systems from the W28 Y1 warhead.

The B28 was deployed by the United States military, equipping Air Force bombers and fighter-bombers, including the B-47, B-52, and F-105, as well as Navy carrier-based aircraft such as the A-3B, A-4, and A-5A. The weapon also served as the warhead for the TM-76 Mace surface-to-surface missile and the GAM-77 Hound Dog cruise missile. Under the NATO nuclear weapons sharing program, the B28 equipped Canadian CF-104 squadrons under a dual-key protocol and was supplied to United Kingdom Royal Air Force Valiant and Canberra squadrons under SACEUR command.

A total of approximately 4,500 units were produced before the system was retired in 1991. The weapon was involved in the 1966 Palomares B-52 crash, which involved four B28FI weapons, and the 1968 Thule Air Base B-52 crash.