The 20,000 ton bomb was 2,000 times more powerful than any bomb ever used before.

Hiroshima was one of the main supply depots for the Japanese army.

The bomb was dropped from the Enola Gay (an American B-29 Superfortress) at 0815 local time.

The bomb was dropped 10 days after the Japanese refused to sign an unconditional surrender, the 'Potsdam declaration'.

President Harry S Truman announced:

"If they do not now accept our terms they may expect a rain of ruin from the air the like of which has never been seen on Earth. Behind this air attack will follow by sea and land forces in such number and power as they have not yet seen, but with fighting skill of which they are already aware."