The B61 nuclear bomb is the primary thermonuclear gravity bomb in the United States Enduring Stockpile following the end of the Cold War. It is a low to intermediate-yield strategic and tactical nuclear weapon featuring a two-stage radiation implosion design. The B61 is of the variable yield ("dial-a-yield" in informal military jargon) design with a yield of 0.3 to 340 kilotons in its various mods. It is a Full Fuzing Option (FUFO) weapon, meaning it is equipped with the full range of fuzing and delivery options, including air and ground burst fuzing, and free-fall, retarded free-fall and laydown delivery. It has a streamlined casing capable of withstanding supersonic flight and is long, with a diameter of about . Basic weight is about , although the weights of individual weapons may vary depending on version and fuze/retardation configuration. As of 2020, it is undergoing a 12th modification. According to the Federation of American Scientists in 2012, the roughly 400 B61-12s will cost $28 million apiece. Development In 1961 a report was issued indicating Navy and Air Force interest in a lightweight bomb to replace existing weapons, but that both services had considerably different military requirements for such a weapon. However, the authors of the report believed that due to advances in technology it was possible for a single weapon to fulfill both requirements. This was followed by a report from Sandia in mid-1962, that believed that a lightweight nuclear bomb with full fuzing option was possible and that such a program could be completed in a short time-span, possibly by 1965 or 1966. Development of the weapon that would become the B61 was authorized in December 1962. The justification for the program was that the new weapon would modernize the nuclear arsenal, improve the capability of aircraft and simplify the nuclear weapons inventory by replacing lower-yield versions of the B28 and B43 nuclear bombs. The desired production date was June 1965. The weapon was designated the TX-61 in January 1963. Due to the short time-scales, it was decided to make maximum use of off-the-shelf components such as those developed for the B57 nuclear bomb. Environmental conditions specified included an indefinite temperature range of to , shocks of up to 40g, and in flight temperatures of up to for up to 40 minutes. Parachute deployment was specified to produce a maximum load of . The weapon was designed and built by the