The Mark 21 nuclear bomb was a United States nuclear gravity bomb first produced in 1955. It was based on the TX-21 "Shrimp" prototype that had been detonated during the Castle Bravo test in March 1954. While most of the Operation Castle tests were intended to evaluate weapons intended for immediate stockpile, or which were already available for use as part of the Emergency Capability Program, Castle Bravo was intended to test a design which would drastically reduce the size and costs of the first generation of air-droppable atomic weapons (the Mk 14, Mk 17 & Mk 24). At 12 feet 6 inches (3.81 m) long, 56 inches (1.42 m) in diameter, and weighing 15,000 pounds (6,800 kg), the Mk-21 was half the length and one-third the weight of the Mk-17/24 weapons it replaced. Its minimum yield was specified at four megatons.
Quantity production of the Mk-21 started in December 1955 and ran until July 1956. Three marks were produced; the Mk-21C was proof tested as the Operation Redwing Navajo shot, with a yield of 4.5 megatons. Starting in June 1957 all Mk-21 bombs were converted to the more powerful Mk-36, which was removed from service in 1962.