Bat bombs were bomb-shaped casings with numerous compartments, each containing a 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 roost in eaves and attics. The incendiaries would start fires in inaccessible places in the largely wood and paper construction of the Japanese cities that were the weapon's intended target.
Developed by the United States during World War II, four biological factors gave promise to this plan. First, bats occur in large number. Second, bats can carry more than their own weight in flight. Third, bats hibernate, and while dormant they do not require food or maintenance. Fourth, bats fly in darkness, then find secluded places to hide during daylight.
The plan was to release bat bombs over Japanese cities having widely-dispersed industrial targets. The bats would spread far from the point of release due to the relatively high altitude of their release, then at dawn they would hide in buildings across the city. Shortly thereafter built-in timers would ignite the bombs, causing widespread fires and chaos. The bat bomb idea was conceived by dental surgeon Lytle S. Adams, who submitted it to the White House in January, 1942, where it was subsequently approved by President Roosevelt.
It was envisioned that ten B-24 bombers flying from Alaska, each carrying a hundred shells packed with bomb-carrying bats could release 1,040,000 bat bombs over the target—the industrial cities of Osaka Bay. A series of tests to answer various operational questions were conducted. In one incident the Auxiliary Army Air Base in Carlsbad, New Mexico, was set on fire when armed bats were accidentally released. The bats incinerated the test range and roosted under a fuel tank. Following this setback, the project was relegated to the Navy in August 1943, who renamed it Project X-Ray, and then passed it to the Marine Corps that December. The Marine Corps moved operations to the Marine Corps Air Station at El Centro, California. After several experiments and operational adjustments, the definitive test was carried out on a mockup of a Japanese city built by the Chemical Warfare Service at their Dugway Proving Grounds test site in Utah.
More tests were scheduled for the summer of 1944 but the program was cancelled by Fleet Admiral Ernest J. King when he heard that it would likely not be combat ready until mid-1945. By that time it was estimated that $2 million had been spent on the project. It is thought that development of the bat bomb was moving too slowly, and was overtaken in the race for a quick end to the war by the atomic bomb project.
Dr. Adams maintained that the bat bombs would have been effective without the devastating effects of the atomic bomb. He is quoted as having said: “Think of thousands of fires breaking out simultaneously over a circle of forty miles in diameter for every bomb dropped. Japan could have been devastated, yet with small loss of life. ”