The development of the Mk 41 began in 1955 with a USAF requirement for a Class B (high-yield, over 10,000 lb/4,545 kg) 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.
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. Two versions were deployed, Y1, a "dirty" version with a tertiary stage encased with U-238, 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, this weapon weighed in at 4850 kg. It remains the highest yield-to-weight ratio of any weapon created. The US claimed in 1963 that it could produce a 35 megaton fusion bomb, and put it on a Titan II (3700 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 1000kg of Lithium 6 Deuteride was used and was contained in a cylinder of natural Uranium with an inner casing of U-238.
The Mk-41 was an example of a fission-fusion-fission type thermonuclear weapon, known by the term "thermonuclear triple threat". Such devices were so called "dirty" H-bombs because when detonated they produce very large amounts of intensely radioactive fallout that often caused illness or death to those with whom it came into contact.
The weapon was 12 ft 4 in (3.76 m) long, with a body diameter of 52 in (132 cm). 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.
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.