The author Robert Heinlein wrote a story about radioactive weapons of mass destruction midway into World War II. He wrote about weapons that would kill people through radioactivity. This was actually a pretty good story. However, he considered writing about an atomic bomb instead but didn't do so because he thought that the U.S. government might be secretly working on such a weapon. In science fiction, the idea of an atomic bomb predates World War II.
After the Trinity test, engineers at Polaroid noticed that radioactivity in the atmosphere was affecting their photographic film, and so they figured out that an atomic bomb had been detonated. Since the light from the Trinity test could be seen from a great distance, the army had a cover story that a secret weapons depot had exploded.
In the early 1930s, Japanese comic books had stories where the Japanese would be victorious against the United States by destroying the American fleet in a surprise attack.