Bat bombs were an experimental World War II weapon developed by the United States. The bomb consisted of a bomb-shaped casing with over a thousand compartments, each containing a hibernating Mexican free-tailed bat with a small, timed incendiary bomb attached. Dropped from a bomber at dawn, the casings would deploy a parachute in mid-flight and open to release the bats, which would then disperse and roost in eaves and attics in a 20–40-mile radius (32–64 km). The incendiaries, which were set on timers, would then ignite and start fires in inaccessible places in the largely wood and paper constructions of the Japanese cities that were the weapon's intended target.
They found that bats can carry almost double their own weight in flight and that their nocturnal behavior made them perfect for sneaking into rooftops and structures undetected. The inventor of napalm served a brief stint on the project and said the bat bombs yielded 30 times the destruction of conventional incendiary bombs.
They were so good at secretly penetrating buildings, that when a few escaped testing, they managed to destroy a fuel tanker, air hangar, and a general’s car. Clearly they could be destructive, but more work was needed to deploy them effectively.