Known as "Little Boy", the bomb contained the equivalent of between 12 and 15,000 tons of TNT and devastated an area of five square miles. More than 60 per cent of the city's buildings were destroyed.
The bomb was dropped from an American B-29 plane, called Enola Gay, at 0815 local time. The crew said they saw a column of smoke rising and intense fires.
Initial Japanese figures put the death toll at 118,661. But later estimates suggest the final toll was about 140,000, of Hiroshima's 350,000 population. Many have also suffered long-term sickness.
Three days later, the US dropped a second, bigger atomic bomb on the city of Nagasaki, killing nearly 74,000. r
Japan surrendered to the Allies on August 14, 1945.