Project X-ray: America's secret WW2 plan to drop bat bombs on Japan

Image by Clément Falize | Unsplash Images

For thousands of years and in a variety of methods animals have found themselves on the field of battle. From elephants to horses, dogs to pigeons, creatures great and small have played a role in human conflict. And over the centuries, mankind has concocted some pretty bizarre ways to deploy animals in combat situations.

One of the most bizarre must surely be the American idea to drop bat bombs over Japan during WWII. You’d be excused for thinking this was just a hare-brained idea that never made it past the ideas table, however, research for the concept went so far as to drain the U.S. government of $2 million (equivalent to $31 million in 2021) with the project going all the way to near deployment.

On December 7, 1941, the might of Imperial Japan descended on the U.S. Pacific Fleet stationed at Pearl Harbor. The infamous attack awoke the sleeping giant who quickly declared war on the ‘land of the rising sun’. Men and women enlisted in their thousands, swearing revenge for the surprise attack.

Amongst the ranks of angry Americans was a dentist from Pennsylvania called Lytle S. Adams who had just returned from a trip to Carlsbad Caverns National Park in New Mexico. Adams had been mesmerised by the sheer number of bats roosting in the caves of the national park and had come away impressed by the creature's strength of flight. It wasn’t long before the dentist decided to try his hat at inventing and concocted an idea that he believed could exact revenge on Japan.

He penned his thoughts in a letter and addressed them to someone he knew in the White House. That acquaintance happened to be the First Lady, Eleanor Roosevelt, who took the idea to her husband President Franklin D. Roosevelt. After liaising with aides, officials, military personnel and scientists Roosevelt decided to give Adam’s suggestion a go. ‘This man is not a nut. It sounds like a perfectly wild idea but is worth looking into,’ a presidential memorandum stated.

So what exactly was this ‘wild idea’? Adams suggested attaching bats with timed incendiary bombs before dropping the animals over various Japanese cities just before dawn. The bats would then roost under the eaves of buildings before exploding. At the time, the majority of Japanese buildings were made of wood, bamboo and paper, so the idea was that the incendiary explosions caused by the bats would unleash huge widespread fires. It was then hoped that the subsequent havoc and destruction would bring Japan to its knees.

‘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,’ Adams would later state.

The U.S. Air Force was delegated to the project and Adams was told to assemble a team. The team's first job was to select the species of bat best suited to the task at hand. The team immediately set to work and began travelling around the United States. 'We visited a thousand caves and three thousand mines,’ Adams recalled. ‘Speed was so imperative that we generally drove all day and night when we weren't exploring caves. We slept in the cars, taking turns at driving. One car in our search team covered 350,000 miles.’

The bat they ended up settling on wasn’t the largest or the strongest but one of the most abundant in North America. It was the Mexican free-tailed bat, a winged mammal that measures 4.1cm and weighs around 13g, the same as a £2 coin. After permission was granted from the National Park Service, thousands of free-tailed bats were captured with nets so the experiment could begin in earnest.

The next challenge the team faced was designing a bomb tiny enough for the bat to carry. The bats were only able to carry a bomb that weighed roughly their own body weight, this meant the team had to pack an explosive punch into a device weighing only 15-18g.

Harvard chemist Louis Fieser had just invented napalm, a highly flammable gel-like liquid that once ignited burned uncontrollably, making it the perfect incendiary weapon to strap to the tiny bats. A celluloid capsule was invented to house the napalm equipped with a time delay fuse; these devices were called H-2 units. The units were then stuck to the bats via an adhesive.

The team then needed to work out how to deploy the bats from an aircraft. They already knew that lowering the temperature around the bats sent them into hibernation, proving to be an effective method of controlling the creatures in transportation. This led to the development of a bomb-shaped casing around 5ft in length that housed numerous cooled circular trays inside. The trays were designed like egg cartons for the tiny bats to be held in. Each bomb could hold up to 1,040 bats.

The bomb carriers were then equipped with a parachute designed to deploy at around a thousand feet above the ground, at which point the sides of the carrier would fall away. The hope was the bats would then have time to wake up from their hibernation and fly off. Upon exiting the carrier, the fuse on their incendiary device would be activated with a 30-minute timer. The idea was to drop them at dawn, a time when bats tended to roost. The creatures would fly off in search of a spot to rest with the Americans hoping that spot was a Japanese building.

Whilst the concept was beginning to sound solid, implementation proved a challenge. Getting the bats to wake up from hibernation was far from easy, many died during testing as they plummeted to the ground still asleep. In one incident, armed bats were accidentally released which led to the burning down of an airbase in New Mexico; proof that the project was onto something but senior officials were beginning to lose patience.

In late 1943, the project was handed to the Marines, who re-labelled it Project X-Ray. After some fine-tuning, successful drops of the bat bombs were conducted over fake Japanese cities created at test sites in Utah.

‘A reasonable number of destructive fires can be started in spite of the extremely small size of the units,’ wrote the chief of incendiary testing. ‘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.

Just when it seemed that the bats were finally ready for deployment, the project was mothballed. Senior officials had decided to throw their weight behind another secret project, the development of the atomic bomb. The Manhattan Project overtook Project X-Ray and the rest as they say is history.