Nuclear fission the Manhattan Project during World War II (September 1, 1939 – September 2, 1945) carried out most of the early scientific work on fission chain reactions, culminating in the three events involving fission bombs that occurred during the war. The first fission bomb, codenamed "The Gadget", was detonated during the Trinity Test in the desert of New Mexico on July 16, 1945. Two other fission bombs, codenamed "Little Boy" and "Fat Man", were used in combat against the Japanese cities of Hiroshima and Nagasaki in on August 6 and 9, 1945 respectively. Even the first fission bombs were thousands of
B61 nuclear bomb Mod 12. With reductions planned of nuclear weapons in Europe, which may retire the older Mod 3 and 4 and the Mod 10 already slated for retirement, and the possibility that European nations may not build or procure new aircraft to carry the Mod 12 would eliminate the need for consolidation into a new type. While the B83 nuclear bomb is not as old as the B61, the Pentagon sees it as a "relic of the Cold War," believing that deploying a megaton-yield gravity bomb, the highest level nuclear weapon left in the U.S. inventory, to Europe is "inconceivable" at