Fortunately for the United States, the impact of World War II was almost solely felt through rationing and news from abroad and rarely were American civilians affected firsthand. As a result, the American home front became a strong source of resources and morale for the soldiers overseas – something that most countries involved in the war didn't have. Realizing this, the Japanese looked for a relatively cheap and safe way to disrupt the American home front.
In 1944, the Japanese decided to tap into a jet stream that they had discovered a few years earlier, one that traveled from Asia to North America at about 30,000 feet. Their plan was to attach bombs to giant balloons and release them into the jet stream, where they'd be carried silently and dispersed across the US at random. Overall, the plan wasn't to kill Americans, but rather to start forest fires in the Pacific Northwest and also instill panic in the population and, in turn, diminish morale on the home front.
The balloons were around 30 feet in diameter and from the top of the balloon to the payload underneath measured around 70 feet. Each balloon bomb consisted of sandbags for ballast that would be released as the balloon descended, allowing it to drop some weight, gain a little altitude, and carry on a bit further. Once the sandbags had all been dropped, four incendiary bombs would be released one at a time until none were left, then a single anti-personnel bomb would fall and the balloon would ignite itself and be destroyed.
Launched in groups, the balloons were at the mercy of the jet stream and typically took a few days to cross the Pacific. Since they were unguided, the balloons had a wide distribution and have been discovered from Alaska to Mexico and from Hawaii to Michigan.
The bombs started arriving in Western US in late 1944 and at first no one really knew what they were. However, geologists sampled the sand and determined that it had come from a small section of beach east of Tokyo. Until the end of the war, the US Government asked media outlets not to report on the bombs in order to prevent the Japanese from tracking whether they were successful. This apparently worked because the Japanese pulled the funding for the bombs after only a few months of launches, assuming that the balloons weren't hitting their targets.
Overall, the bombs were not successful; less than 300 out of an estimated 9,000 have ever been found – around 3%. However, one bomb detonated in rural Oregon in spring 1945 and killed a pregnant woman and five children, making them the only American civilians killed in the US as a result of enemy action.
After the war, talk of the bombs began to spread and it was found that seven landed in Nebraska including one in Omaha, one landed ten miles from Detroit and another landed near Grand Rapids. Balloon bombs, while largely unsuccessful, continued to be discovered throughout the US and Canada following the war and one was even discovered in British Columbia in October 2014.