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
Mark 4 nuclear bomb Mark 4 nuclear bomb The Mark 4 nuclear bomb was an American nuclear bomb design produced starting in 1949 and in use until 1953. The Mark 4 was based on the earlier Mark 3 Fat Man design, used in the Trinity test and the bombing of Nagasaki. The Mark 3 was essentially handmade and constituted an emergency wartime expedient design; the Mark 4 utilized essentially the same basic principles (materials, dimensions of the nuclear core and explosive components) but reengineered the overarching system to be safer and easier to produce. The basic idea was to "GI-proof" otherwise sensitive nuclear weapons.