The first bomb (Hiroshima) was named Little Boy, and the second bomb (Nagasaki) was named Fat Boy.
The Hiroshima bomb was long, compared to its circumference, and was named after FDR. The Nagasaki bomb was wide in its circumference, compared to its  length, and was named after Churchill.
The two bombs used two different fissile materials, and two different methods to achieve critical mass.