Between November 1944 and April 1945, the Japanese launched about 9,300 balloons from sites on Honshu, of which about 300 were found or observed in the U.S., with some in Canada and Mexico.[1] The balloons traveled on high-altitude and high-speed currents over the Pacific Ocean, today known as the jet stream, and used a sophisticated ballast system to control altitude on their three-day flight. Despite high hopes for the operation, the bombs were largely ineffective due to damp conditions and malfunctions, causing only minor damage and six deaths (from a single civilian incident in Oregon in May 1945). The Fu-Go balloon bomb was the first weapon to possess intercontinental range, with its flights being the longest-ranged attacks in the history of warfare at the time.[2]
https://en.wikipedia.org/wiki/Fu-Go_balloon_bomb