The Little Boy bomb was exceptionally simple, and built with redundancy in mind. The basis is very simple: take two chunks of fissile material, each formed to be less than critical mass for its particular shape. read more
The United States detonates the world's first atomic bomb at a test site in New Mexico on July 16, 1945. Less than a month later, atomic bombs were dropped on the Japanese cities of Hiroshima and Nagasaki. read more
The Nagasaki bomb, at least, was an implosion model, and these had been not only continued to be tested after the war (the Operation Crossroads weapons were essentially Fat Man devices), but were the subject of on-going interest and development. The Hiroshima bomb, Little Boy, was a model that was obsolete even as it was being dropped. read more