During World War II, the Japanese developed a system of bombing the U.S. by unmanned balloons made of paper sent thousands of miles across the Pacific. It sounds as crazy as the bat bombs the U.S. Navy developed. But the atom bomb was once a crazy idea, too.
Through experimentation, Japanese researchers found that at just over 30,000 feet altitude, the jet-stream could carry a large balloon about 5,000 miles across the Pacific in three days in late autumn when the stream was strongest.
They then developed an extremely clever, and simple, mechanical device to automate the flight of the balloons and release the explosives. To prevent too much fluctuation in altitude as the temperature changed in the night vs. the day, engineers created a system controlled by barometric sensors. If the altitude got too low, below 30,000 ft, a small charge would fire, ejecting two sandbags mounted on a spoked wheel containing other sandbags and the explosive devices themselves. When the temperature heated up in the day time and the balloon rose above 38,000 feet, barometric-controlled valves would automatically open, releasing hydrogen and thus lowering the balloon to the desired level.
The control system was set up to last just three days at which point (in theory) there would be no sandbags left, just the incendiary devices ready to be released once the balloon dipped below 30,000 ft. At this point, the balloon would theoretically be over the U.S. and incendiary devices ranging from 5 kg to 15 kg would be released. A fuse would also be lit which would burn for approximately 84 minutes before igniting the balloon itself with its 19,000 or so cubic feet of hydrogen then exploding.
Could this weird system actually work? It indeed did, but not as well as the Japanese had hoped. Still, there were American casualties of the balloon bombs. Read what happened at Gizmodo.