Hi altairz,
Einstein's input on the atomic bomb was the old chestnut E=mc
2, the principle is clearly shown by the atomic bomb in the sense that he says a large amount of energy can be released from a small amount of matter, of course it's worth mentioning, being a strong pacifist, that I'm sure he never had the atomic bomb or anything along these lines in mind when he came up with this - of course until Hitler (different story/thread).
In 1938 a couple of German chaps called Fritz Strassmann and Otto Hahn split a uranium atom, this along with the state of Germany at the time led to discomfort among a group of lesser known physicists who went to Einstein who was convinced of the problem signed a letter addressed to the President of America at the time (Roosevelt), leading to the
http://en.wikipedia.org/wiki/S-1_Uranium_Committee#Briggs_Advisory_Committee_on_Uranium", which was to study the reactions of uranium, progress was slow, very slow, until the British did a report which said an atomic bomb could be ready to use in 1943, this prompted more research to be conducted, cue the Manhattan Project.
So it was more the fact that nuclear fission was discovered which lead to the realisation and fear of the potential of such weaponry that sparked the bombs to be designed, of course it was Einstein's principle which lead to the logic behind the bombs from this discovery.
Hope this clears your question up a little.