What was the name of the atomic bomb dropped by America on Hiroshima?

The bombs dropped by the US on Hiroshima (the bomb was codenamed ‘Little Boy’) and Nagasaki (code-named ‘Fat Man’). Yet, these bombs were very small in their destructive capacity (measured in terms of kiloton yield) as compared to the nuclear bombs that were to be available in the stockpiles assembled by the superpowers. The yield of Little Boy and Fat Man were 15 and 21 kilotons respectively. By the early 1950s the US and the USSR were already making thermonuclear weapons that had a yield between 10 and 15 thousand kilotons. In other words, these bombs were a thousand times more destructive than the bombs used in Hiroshima and Nagasaki.