The Federation of American Scientists believes that there are some 200 B61 thermonuclear gravity bombs scattered across these four countries. Under a NATO agreement struck during the Cold War, the bombs, which are technically owned by the U.S., can be transferred to the control of a host nation’s air force in times of conflict
Total production of all versions was approximately 3,155, of which approximately 1,925 remain in service as of 2002, and some 1,265 are considered to be operational
The B61 has been deployed by a very wide variety of U.S. military aircraft. Aircraft cleared for its use have included the B-58 Hustler, B-1, B-2, B-52, and FB-111 strategic bomber aircraft; the F-100 Super Sabre, F-104 Starfighter, F-105 Thunderchief, F-111 and F-4 Phantom II fighter bombers; the A-4 Skyhawk, A-6 Intruder, and A-7 Corsair II attack aircraft; the F-15 Eagle and F-15E Strike Eagle; F22 Raptor; British, German and Italian Panavia Tornado IDS aircraft, plus Belgian and Dutch F-16 Fighting Falcon can also carry the B61.
Though exact numbers are hard to establish, research done by the Natural Resources Defense Council suggests approximately 480 are deployed with United States Air Force units in various European countries
The B61 is a variable yield bomb designed for carriage by high-speed aircraft. It has a streamlined casing capable of withstanding supersonic flight speeds. The weapon is 11 ft 8 in (3.58 m) long, with a diameter of about 13 in (33 cm). Basic weight is about 700 lb (320 kg), although the weights of individual weapons may vary depending on version and fuze/retardation configuration.
The newest variant is the B61 Mod 11, a hardened penetration bomb with a reinforced casing (according to some sources, containing depleted uranium) and a delayed-action fuze, allowing it to penetrate several metres into the ground before detonating, damaging fortified structures further underground . The Mod 11 weighs about 1,200 lb (540 kg). Developed from 1994, the Mod 11 went into service in 1997 replacing the older megaton-yield B53 bomb, a limited number of which had been retained for anti-fortification use. About 50 Mod 11 bombs have been produced, their warheads converted from Mod 7 bombs. At present, the primary carrier for the B61 Mod 11 is the B-2 Spirit.
Most versions of the B61 are equipped with a parachute retarder (currently a 24-ft (7.3 m) diameter nylon/Kevlar chute) to slow the weapon in its descent, giving the aircraft a chance to escape the blast (or to allow the weapon to survive impact with the ground in laydown mode). The B61 can be set for airburst, ground burst, or laydown detonation, and can be released at speeds up to Mach 2 and altitudes as low as 50 feet (15 m). Fusing for most versions is by radar.
The B61 is a variable yield, kiloton-range weapon called “Full Fuzing Option”(FUFO) or “Dial-a-yield” by many service personnel. Tactical versions (Mods 3, 4, and 10) can be set to 0.3, 1.5, 5, 10, 60, 80, or 170 kiloton explosive yield (depending on version). The strategic version (B61 Mod 7) has four yield options, with a maximum of 340 kilotons. Sources conflict on the yield of the earth-penetrating Mod 11; the physics package or bomb core components of the Mod 11 are apparently unchanged from the earlier strategic Mod 7, however the declassified 2001 Nuclear Posture Review  states that the B-61-11 has only a single yield; some sources indicate 10 KT, others suggest the 340 kiloton maximum yield as the Mod-7.