The B-41 (also known as Mk-41) 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 . A top secret document (DCI Briefing to the JCS, 30 July 1963), states “The US has stockpiled bombs of 9 MT and 23 MT...” which would likely be referring to the B-41's actual yield(s). The B-41 was the only three-stage thermonuclear weapon fielded by the U.S. History In June 1955, the US Department of Defense requested a feasibility study for a Class B (over weight) bomb and warhead. By summer of 1956, US Air Force Strategic Air Command produced a requirement for a Class B bomb, while the DoD produced a requirement for a Class B warhead. The University of California's Radiation Laboratory (now Lawrence Livermore National Laboratory) proposed the use of the existing Bassoon device that was test fired in the Zuni and Tewa shots of Operation Redwing. Stockpiling of this new weapon was planned for January 1959. Dirty and clean (low fission fraction) versions of the device were proposed, with the clean version being dependent on a nuclear test in Operation Hardtack I. The nomenclature of TX-41 and XW-41 was assigned to the weapon in November 1956, and in December plans were made to conduct drop tests of the weapon from the B-47 bomber. A laydown version of the bomb was requested, however development of such a weapon would add 1 to 2 more years to its development. A non-laydown weapon was subsequently requested. The weapon's military characteristics were approved in February 1957. The weapon was to be carried by the B-47, B-52, B-66 and systems 110A and 125A. The bomb would be able to withstand without damage the various flight stresses. Fuzing would include contact and air-burst modes, and would be selectable in flight. Contact fuzing would act as a backup for airburst fuzing and a parachute to slow the rate of fall would be developed. Compatibility with the Navaho missile and B-58 bomb pod was also requested. By March 1957, it was decided to place equal emphasis on the clean and dirty versions of the weapon. By this point the weapon was to be in diameter, with a warhead length of and weight of , while the bomb was to have a length of and