Do you mean the atomic bomb? The answer is no, it was kept secret from both the Japanese and the American people, and the US did not warn the Japanese about it (contrary to much mistaken Internet opinion).

If you mean the firebombing campaign, the US did give the Japanese some indication that it was bombing cities indiscriminately (though I am not sure they warned them explicitly prior to the firebombing of Tokyo in March 1945, which was the most deadly bombing raid of the war), in an effort to get the Japanese civilians to abandon their posts. After March 1945, any American could read about the bombing of Tokyo and by late summer there were American newspapers keeping track of which cities had been named as potential incendiary raid targets and which ones had already been burned.