 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.