The first atom bomb to be used aggressively in battle, was dropped on Hiroshima, Japan by the United States at 8:16 a.m. on 6 August 1945. It had an explosive power equivalent to that of around 15 kilotons of trinitrotoluene (C7H5O6N3), called TNT.

Code-named Little Boy, it was 3.04 m (10 ft) long and weighed 4,082 kg (9,000 lb). It burst 565 m (1,850 ft) above the city centre.