B41 nuclear bomb The B41 was a thermonuclear weapon deployed by the United States Strategic Air Command in the early 1960s . It was the most powerful nuclear bomb ever developed by the United States , with a maximum yield of 25 megatons . This project came about during the Cold War . Development . The development of the Mk 41 began in 1955 with a USAF requirement for a Class B ( high - yield , over ) weapon . It was based on the " Bassoon " test device first fired in the Redwing Zuni test of 27 May 1956 . An ICBM warhead version of the weapon was cancelled in 1957 . Composition . The Mk 41 was the only three - stage thermonuclear weapon fielded by the U . S . It had a deuterium - tritium boosted primary , probably with lithium - 6 - enriched deuteride fuel for the fusion reaction in the secondary stage . This was followed by a yet - larger third fusion stage , the tertiary stage , compressed by the secondary stage . Finally , there was a fission jacket . Two versions were deployed , Y1 , a " dirty " version with a tertiary stage encased with U - 238 ( natural uranium ) , and Y2 , a " clean " version with a lead - encased tertiary . It was the highest - yield nuclear weapon ever deployed by the United States , with a maximum yield of 25 megatons ( Mt ) , and weighing in at 4 , 850 kg . It remains the highest yield - to - weight ratio of any weapon created . The US claimed in 1963 that it could produce a 35 Mt fusion bomb , and put it on a Titan II ( 3 , 700 kg payload ) , almost doubling the yield - to - weight ratio of the B - 41 . The Mk 41 was of the usual long cylindrical shape and weighed 10 , 670 lb ( 4 , 840 kg ) . The nuclear fusion warhead was of the Teller - Ulam type and used a 40 – 100 kiloton implosion type nuclear fission primary fueled by HEU to trigger the lithium - 6 deuteride fusion fuel . Between 500 and 1 , 000 kg of lithium deuteride was used and was contained in a cylinder of natural uranium ( U - 238 ) with an inner casing of U - 235 . The Mk 41 was an example of a fission - fusion - fusion - fission type thermonuclear weapon , or tertiary stage bomb . The additional tertiary fusion stage , compressed by a previous fusion stage , could be used to make a bomb with yields as large as desired ( see Tsar Bomba , a Soviet three - stage bomb and the highest - yield nuclear weapon ever built or tested ) . Physical characteristics . The weapon was 12 ft 4 in ( 3 . 76 m ) long , with a body diameter of 4 ft 4 in ( 1 . 32 m ) . It weighed 10 , 670 lb ( 4 , 850 kg ) . It was carried only by the B - 52 Stratofortress and B - 47 Stratojet . It could be deployed in free - fall or retarded ( parachute ) configuration , and could be set for airburst , groundburst , or laydown delivery . Service life . The Mk 41 ( designated B41 from 1968 on ) entered service in 1961 . About 500 of these weapons were manufactured between September 1960 and June 1962 . The Mk 41 was progressively phased out of service from 1963 in favor of the B53 nuclear bomb . The last B41s were retired in July 1976 . Efficiency . During its operation , the Mk - 41 was the most efficient known thermonuclear weapon in terms of yield to actual weight , with a 5 . 2 Megaton / tonne ratio ( based on a 25 Mt yield ) . Its blast yield was 25 to 50 % that of the AN602 Tsar Bomba , which delivered a blast of 50 or 100 megatons of TNT , depending on its own configuration as a " clean " ( lead encased ) or dirty ( uranium encased ) bomb . However even at the Tsar Bomba ' s theoretical maximum yield of 100 megatons , it would still only achieve a yield to weight ratio of ~ 3 . 7 Megaton / tonne , thus the Mk - 41 is the most efficient , highest yield to weight ratio , weapon ever created .