From Wedge, a history of the conflict between the FBI and the CIA:

One famous undertaking was spawned by an unsolicited letter to the president from a Mr. Adams of Irwin, Pennsylvania, asserting that the Japanese were deathly frightened of bats and suggesting that America consider the opportunities for “frightening, demoralizing, and exciting the prejudices of the people of the Japanese empire” by a “surprise attack” in which Japan would be bombed with live specimens. The president passed Adams’ letter to Wild Bill [Donovan, head of the COI] with a note asserting that “this man is not a nut.” Donovan promptly commissioned the curator of mammals at the American Museum of Natural History to work with the Army Air Corps; bats were strapped into catapults and flung, and their trajectories noted on clipboards, but the project was terminated when it was discovered that bats would freeze to death at forty thousand feet. Also, though no one had bothered to check Adams’ assertion at the time, it turned out that the Japanese did not fear bats.