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 ("modifications"). It is a Full Fuzing Option (FUFO) weapon, meaning it is equipped with several fuzing options including air burst, ground burst, free-fall, retarded free-fall (parachuted) and laydown delivery. It has a streamlined casing allowing its release by supersonic aircraft. It is long, with a diameter of about ; the basic weight of the B61 is about , with weight differences for parachute delivery. As of 2020, the weapon was undergoing a 12th modification. In 2012, the roughly 400 B61-12s were estimated to each cost "more than its weight in gold" – $28 million apiece.

, approximately 505 B61s are in service. The strategic B61-11 nuclear bunker buster is believed to be the only nuclear weapon assigned to the B-2 Spirit stealth strategic bomber, which can carry up to sixteen. The B61-12 is the only remaining US tactical nuclear weapon, with 200 stockpiled, approximately 100 in the US, and another 100 in certain European NATO member states under its nuclear sharing arrangement, maintained by munitions squadrons. The B61-12 is certified for delivery by the F-35A Lightning II stealth fifth generation fighter, and F-15E Strike Eagle, F-16 Fighting Falcon, and Panavia Tornado IDS fourth generation fighters. Alongside the United States Air Force, the air forces of Belgium, Germany, Italy, the Netherlands, Turkey, and the United Kingdom are believed to host B61-12 bombs, and operate a mixture of the certified aircraft to deliver them. The B61-13 is being developed as a strategic bunker buster for the forthcoming US B-21 Raider strategic bomber.

B61 variants have been previously deployed on many U.S. aircraft. Such as the B-52 Stratofortress and B-1 Lancer strategic bombers; F-117 Nighthawk and F-111 Aardvark ground-based attack aircraft, and F/A-18 Hornet and A-7 Corsair II carrier-based attack aircraft.

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 , shocks of up to , 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 Los Alamos National Laboratory in New Mexico. Production engineering began in 1965, with the first war-reserve B61-0 weapon accepted by the AEC in December 1966. However production was halted in May 1967 for design modifications to be made before resuming in January 1968. Shot Flintlock Halfbeak in June 1966 may have been a full yield test of the weapon with other tests conducted between 1963 and 1968 at the Nevada Test Site.

During testing, spin motors were added to the weapon to produce a spin of 5 revolutions per second. This was to improve stability and decoupling during high speed delivery. Three alternatives to the weapon were also explored for still-classified reasons. The first alternative was halted due to its similarity to the original TX-61 design, and the Department of Defense was unfavorable to the second option which halted its exploration. The TX-61 design was ordered to "proceed along the lines" of the third alternative in May 1964, though it is unclear what that means.

Total production of all versions was approximately 3,155, of which approximately 540 remain in active service, 415 in inactive service and 520 are awaiting dismantlement .

Fourteen versions of the B61 have been designed, known as Mod 0 through Mod 13 (see table below). Each shares the same physics package, with different yield options. One variant is the Mod 11, deployed in 1997, which is a ground-penetrating bunker busting weapon. The Russian Continuity of Government facility at Kosvinsky Kamen, finished in early 1996, was designed to resist US earth-penetrating warheads and serves a similar role as the American Cheyenne Mountain Complex. The timing of the Kosvinsky completion date is regarded as one explanation for US interest in a new nuclear bunker buster and the declaration of the deployment of the Mod 11 in 1997: Kosvinsky is protected by about of granite.

The B61 unguided bomb should not be confused with the MGM-1 Matador cruise missile, which was originally developed under the bomber designation B61.

Deployment

The B61 has been deployed by a variety of US military aircraft. US aircraft cleared for its use have included the B-1 Lancer, B-2 Spirit, B-52 Stratofortress, F/A-18 Hornet, A-6 Intruder, A-4 Skyhawk, F-111, F-15E Strike Eagle, F-16 Fighting Falcon and the F-35A. As part of NATO Nuclear Weapons Sharing, German and Italian Panavia Tornado aircraft can also carry B61s. The B61 can fit inside the F-22 Raptor's weapons bays.

B61 tactical variants are deployed with NATO allies in Europe as part of the NATO Nuclear Weapons Sharing Program."The Bulletin of the Atomic Scientists", 26 October 2013 About 150 bombs are stored at six bases: Kleine Brogel in Belgium, Büchel Air Base in Germany, Aviano and Ghedi Air Base in Italy, Volkel Air Base in the Netherlands and Incirlik in Turkey. In 2012, NATO agreed to improve the capabilities of this force with the increased accuracy of the Mod 12 upgrade and the delivery of the F-35 aircraft.Kristensen, Hans M. "New Report: US and Russian Non-Strategic Nuclear Weapons." Federation of American Scientists, 2 May 2012. This added a modest standoff capability to the B61. The F-35A was certified to carry the B61-12 in October 2023, marking the first time an American fifth generation fighter has been certified as nuclear capable, and the first new platform in the NATO inventory to achieve such status since the early 1990s.EXCLUSIVE: F-35A officially certified to carry nuclear bomb. Breaking Defense. 8 March 2024.

In June 2025, the UK announced that it will purchase 12 F-35A fighter jets and formally join NATO’s Dual Capable Aircraft (DCA) nuclear sharing mission and will also buy the B61-12. The F-35A, a conventional take-off variant, is nuclear-capable and interoperable with US B61-12 gravity bombs, which already forms the core of NATO’s air-delivered nuclear capability.

Design

The B61 is a variable yield ("dial-a-yield" colloquially) dual use tactical and strategic bomb equipped with Full Fuzing Option (FUFO) designed for external carriage by high-speed aircraft. It has a streamlined casing capable of withstanding supersonic flight. The original B61-0 weapon was long, with a diameter of and a basic weight of with most later weapons having approximately the same dimensions and weight, except for the Mod 11 version which has a weight of approximately .

Ground operations

The B61 is armed by ground-based personnel via an access panel located on the side of the bomb, which opens to reveal nine dials, two sockets, and a T-handle which manually triggers the "command disable" function. One of the sockets is for a MC4142 "strike enable" plug which must be inserted in order to complete critical circuits in the safety/arming and firing mechanisms. The other socket is the PAL connector, which has 23 pins marked with alphabetic letter codes.

The B61 "command disable" mechanism functions as follows: after entering the correct three-digit numeric code it is then possible to turn a dial to "DI" and pull back a T-shaped handle which comes away in the user's hand. This action releases a spring-loaded firing pin which fires the percussion cap on an MC4246A thermal battery, powering it up. Electrical power from the thermal battery is sufficient to "fry" the internal circuitry of the bomb, destroying critical mechanisms without causing detonation. This makes the bomb incapable of being used. Any B61 which has had the command disable facility activated must be returned to Pantex for repair.

Fuzing and delivery

The B61 can be set for airburst or groundburst detonation, and by free fall, retarded free fall or laydown delivery through the use of a parachute to slow down the weapon during release from the delivery aircraft. Only the Mod 0 to 10 versions of the B61 are equipped with a parachute retarder (currently a 24-ft (7.3 m) diameter nylon/Kevlar chute). This offers the aircraft a chance to escape the blast in its retarded delivery modes, or allows the weapon to survive impact with the ground in laydown delivery mode. Contact preclusion can also be selected by the pilot. The weapon can be released at speeds up to Mach 2 and altitudes as low as . In one of the weapon's laydown modes, it detonates 31 seconds after weapon release.

The Mod 11 is a hardened penetration bomb with a reinforced casing and a delayed-action fuze; this allows the weapon to penetrate several metres into the ground before detonating, damaging fortified structures further underground. Developed from 1994, the Mod 11 went into service in 1997 replacing the older megaton-yield B53 bomb. About 50 Mod 11 bombs have been produced, their warheads converted from Mod 7 bombs. At present, the primary carrier for the Mod 11 is the B-2 Spirit.

Variants

the B61 has 14 variants, referred to as Mod 0 through Mod 13.