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. The United States Navy took control in August 1943, using the codename Project X-Ray.
A series of tests to answer various operational questions were conducted. In one incident, the Carlsbad Army Airfield Auxiliary Air Base (32°15′39″N 104°13′45″W) near Carlsbad, New Mexico, was set on fire on May 15, 1943, when armed bats were accidentally released. The bats roosted under a fuel tank and incinerated the test range.