Who were the atomic bombs dropped on Japan named after?
Nobody. They were called Fatman (the Plutonium implosion MK-III bomb) and Little Boy (the Uranium gun MK-I bomb) because of their shapes.
There is some mythology that the codenames Little Boy was named after FDR and Fatman was named after Winston Churchill to mislead spies into thinking that the "Silverplate" modified B-29s were being modified to transport those leaders, not a new type of bomb. But this is contradicted by the codename Thin Man (the Plutonium gun MK-II bomb) whose design was canceled before beginning work on the Fatman design. Thin Man would have fit FDR much better than Little Boy did and Little Boy could not have been named for Winston Churchill.