Those early B-61s had dials that could make them low-, medium- or high-yield weapons, with explosive power that could be less than, or far greater than, the Hiroshima atomic bomb. Beginning in 1985, a newer B-61-7 replaced the old ones. This strategic version has yields that range from 10 kilotons to more than 300 kilotons, the equivalent of 300,000 tons of TNT. Hiroshima was 12.5 kilotons.
More than 150 of the B-61s -- the 7s and 11s -- are now stored in Western European countries.
The original B-61 entered the stockpile in 1968 as a tactical nuclear bomb. In those days, and for almost two decades, they were hung from F-4 fighter bombers -- on the perimeter of the Soviet Union -- with both U.S. and foreign pilots on 15-minute alert.