On 16 July 1945, a nuclear weapon was detonated successfully for the first time in a test at the White Sands Proving Grounds, about 250 km south of Los Alamos. The bomb used plutonium as its nuclear fuel material and possessed an explosive yield equivalent to 21 kilotons of TNT. The four-tonne uranium bomb, named "Little Boy", was ready to use.
The United States had invested two billion US dollars developing the bomb. President Harry Truman gave the command to drop the new weapon while he was in Germany, negotiating with other victorious Allied countries at the Potsdam Conference.
On 6 August 1945 at 8:16 am the first bomb was dropped without warning over Hiroshima in Japan. "Little Boy" exploded at an altitude of 600 metres directly above the centre of the city. 43 seconds later the enormous pressure wave had completely destroyed three quarters of the Japanese city.
We look at images of that first blast, as well as later atomic bomb tests in the 1940s and 50s.