The atomic bomb (also known as the A bomb) was a nuclear fission device dropped from a B-22 Superfortress similar to one shown below.
The Enola Gay was the aircraft on the mission to bomb Hiroshima which dropped the payload. It was captained by Colonel Paul Tibbets.
Below is a diagram of an A-Bomb similar to the ones dropped on Hiroshima and Nagasaki.
While the explosions at Hiroshima and Nagasaki brought incomprehensible damage to the city's, those bombs dropped in 1945 are no where near as powerful as the nuclear bombs that many of the world powers have today. It is estimated that the “little boy” a-bomb dropped in 1945 had the equivalent of 15,000 tons of TNT. Today's atomic bombs have the equivalent of 50,000,000 tons of TNT.
Top Image from"CNN,"Artist Crew of Enola Gay, Date 1945