From Wired:
Bat Bombs
Toward the end of World War II, the Air Force was looking for a better way to burn Japanese cities to the ground. A dental surgeon contacted the White House, and suggested strapping small incendiary devices to bats, loading them into cages shaped like bombshells and dropping them over a wide area.
According to the plan, millions of bats would escape from the bombshells as they parachuted toward earth, and the flying mammals would find their way into the attics of barns and factories, where they would rest until the charges they were carrying exploded. In the early 1940s, a test with some armed bats went awry, and they set fire to a small Air Force base in Carlsbad, New Mexico. After that accident, the project was turned over to the Navy, which continued it for more than a year. During that time, the Marines conducted a successful proof of concept at Dugway Proving Grounds in Utah, where they released bats over a mock-up of a Japanese city. The critters were able to start quite a few fires.
More here.