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.
The bat bomb was thought up by a dental surgeon from Irwin, Pennsylvania named Lytle S. Adams, an acquaintance of First Lady Eleanor Roosevelt. The inspiration for Adams' suggestion was a trip he took to Carlsbad Caverns National Park, which is home to many bats. Adams wrote about his idea of incendiary bats in a letter to the White House in January 1942, little more than a month after the attack on Pearl Harbor. Adams was intrigued by the strength of bats and knew that they roosted before dawn. He also knew that most of the buildings in Tokyo were constructed of wood instead of concrete. He believed that if time-release incendiaries could be attached to bats, some kind of container holding them could be dropped over the city after dark and the bats would simply roost and burn Tokyo to the ground. The plan was subsequently approved by President Roosevelt. In his letter, Adams stated that the bat was the "lowest form of animal life", and that, until now, "reasons for its creation have remained unexplained". Of Adams, Roosevelt remarked, "This man is not a nut. It sounds like a perfectly wild idea but is worth looking into."
After Roosevelt gave the project his approval, it was relegated to the authority of the United States Army Air Force. Adams assembled the workers for the project, including the mammalogist Jack von Bloeker, actor Tim Holt, and a former gangster, among others. Von Bloeker and his assistant Jack Couffer, self-described "bat lovers", noted that it did not occur to them to question the "morality or the ecological consequences of sacrificing a few million bats". For the duration of the project, many members enlisted in the Air Force, where Adams quickly promoted them to "acting" non-commissioned officers. The team had to determine several variables to make the project feasible, including what kind of incendiaries could be attached to the bats, as well as the temperatures at which to store and transport them. They also had to decide what species of bat to use for the bombs. After testing several species, the Mexican free-tailed bat was selected. Adams had to ask for permission from the National Park Service to harvest large numbers of Mexican free-tailed bats from caves on government property. While the original plan was to arm the bats with white phosphorus, American chemist Louis Fieser joined the team and white phosphorus was replaced with Fieser's own invention, napalm.
Tests were used to determine how much napalm an individual bat could carry, determining that a 14 g (0.5 oz) bat could carry a payload of 15–18 g (0.53–0.63 oz). The napalm was stored in small cellulose containers dubbed "H-2 units". After trying several attachment methods, they decided to attach the H-2 unit to the bats using an adhesive, gluing them to the front of the bats. The bomb carrier was a sheet metal tube approximately 1.5 m (5 ft) in length. The inside of the tube was fitted with twenty-six circular trays, each of which was 76 cm (30 in) in diameter. In total, each bomb carrier could hold 1,040 bats. It was planned that the carrier would be deployed from an airplane, descending to an altitude of 1,200 m (4,000 ft) before deploying parachutes. The sides of the bomb carrier would then fall away, allowing the bats to disperse.
A series of tests to answer various operational questions were conducted. In one incident, the Carlsbad Army Airfield Auxiliary Air Base near Carlsbad, New Mexico, was set on fire on May 15th, 1943, when armed bats were accidentally released The bats roosted under a fuel tank and incinerated the test range. 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 the "Japanese Village", a mockup of a Japanese city built by the Chemical Warfare Service at their Dugway Proving Grounds test site in Utah. Observers at this test produced optimistic accounts. The chief of incendiary testing at Dugway wrote, "A reasonable number of destructive fires can be started in spite of the extremely small size of the units. The main advantage of the units would seem to be their placement within the enemy structures without the knowledge of the householder or fire watchers, thus allowing the fire to establish itself before being discovered".
More tests were scheduled for mid-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 (equivalent to $19 million today) 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. Adams maintained that the bat bombs would have been effective without the devastating effects of the atomic bomb: "Think of thousands of fires breaking out simultaneously over a circle of forty miles [64 km] in diameter for every bomb dropped. Japan could have been devastated, yet with small loss of life." The infamous "Invasion by Bats" project was afterwards referred to by Stanley P. Lovell, director of research and development for Office of Strategic Services (OSS), whom General Donovan ordered to review the idea, as "Die Fledermaus Farce".
