On November 3rd, 1944, Japanese forces launched 9000 balloons carrying 15-kilogram incendiary bombs from Honshu Island east of the Japanese mainland, over the Pacific Ocean over the Western United States. These were called Fu-Go bombs and were considered revenge for the Doolittle Raids. Over a thousand of these landed in the US in 16 states, and in Canada and in Alaska.
On May 5th, 1945, one day before Hiroshima, a woman and her five children were killed by one of these Fu-Go bombs, the only known casualties of the attacks. Elie Mitchell and her daughters were attending a church picnic in Lakeview Oregon when one of them discovered it and picked up a bomb. There it exploded and killed them all. The news about these weapons was suppressed.
Their method of travel and detonation was ingenious. The balloons were released and climbed to 35,000 feet, when the gas in the balloon leaked out causing it to drop altitude. 80 to 100 mph winds moved the balloons eastward over the US coast. When the balloons dropped altitude, a barometric trigger released a sand ballast bag. So up and down they traveled in and out of the jetstream. Once they were over land, it was calculated that the sand ballasts would be gone and so the incendiary bombs themselves would then drop and explode. Some of them were observed to explode in mid air. A few of them made it as far east as Texas. The first one were found by woodcutters who brought it in and the type of sand in the bags revealed to researchers that they were from Japan. Obviously, the psychological effect of pilotless bombs killing randomly would have been devastating.
Then again, we know how this all ended.
Sources: Dick Whitaker (FDick’s Blog), Wikipedia, http://www.stelzriede.com