The first Atomic Bomb to be used in anger was dropped on Hiroshima by the B29 'Enola Gay' on August 6th 1945 killing over 60,000 people and injuring a similar number.
The bomb was codenamed Little Boy. It was a Uranium based device that yielded the equivalent of around 15 kilo tonnes of conventional explosives. It was a simple 'gun' type device being detonated by firing one piece of uranium into another at high speed. However it was considered to be inherently unsafe due to the possibility of accidental detonation and the design wasn't used again.