Leo Szilard [1] is generally credited as coming up with the concept of a fission bomb. He enlisted the help of Einstein, who had political clout, to bring it to the attention of the US government [2].
Sorry, I should have appended an "/s" to my comment. Einstein is associated in a loose fashion, and therefore by name recognition often credited by laypeople-- to creating the atom bomb, but has much less to do with it than many other people apart from theoretical groundwork related to the energies involved in mass. (to greatly simplify things)
[1] https://en.wikipedia.org/wiki/Leo_Szilard [2] https://en.wikipedia.org/wiki/Einstein%E2%80%93Szil%C3%A1rd_...