In December 1941, a Pennsylvania dentist named Lytle Adams sent a letter to the White House with an unusual proposal: use bats as the delivery mechanism for incendiary bombs against Japanese cities. President Roosevelt, who Adams knew personally, forwarded the letter to military research channels with a note calling it "a perfectly wild idea" — but also ordering it be looked into.
The logic was not entirely crazy. Japanese cities in 1941 were built largely of wood and paper. A bomb too small to damage a concrete building could set a wooden structure ablaze. Bats — specifically Mexican free-tailed bats from caves in Texas and New Mexico — roosted in colonies of millions, were lightweight, and most importantly, had an instinct to hide in small dark spaces (like attics and eaves) at dawn when the animal-bomb would be released.
By 1943, the project (codenamed Project X-Ray) had a working design. Each bat would be fitted with a tiny celluloid capsule of napalm with a time-delayed igniter, attached to its chest with a surgical clip. Thousands of bats would be loaded into special bomb casings with individual compartments, the casings dropped over cities, opening at altitude and releasing clouds of bats that would scatter and roost in buildings before the timers went off.
In a test at Carlsbad Army Airfield, New Mexico, things went catastrophically wrong. Some bats carrying live incendiaries escaped from their containers before the test began. They roosted in the base's fuel storage, under the eaves of the general's car, and in a newly constructed auxiliary building. The subsequent fires burned down both the general's car and the building.
Despite this, testing continued. By 1944, the project had spent $2 million and produced promising results. A mock Japanese village was constructed and successfully set ablaze. But by then, the Manhattan Project had absorbed priority resources and attention. Project X-Ray was cancelled.
Historians who have studied both programs note an irony: Project X-Ray might actually have caused more total damage to Japanese cities per dollar spent than the atomic bomb, which cost $2 billion. But unlike the A-bomb, bats would never have ended the war quickly. The project occupies a footnote in history — the strangest weapon that almost was.