As the Nazis were lobbing V2 rockets over the English Channel, the Japanese were fashioning their own "vengeance weapons" as well. Military planners, who were unable to develop an intercontinental missile, instead came up with the idea of balloon bombs.
To make it work, the Japanese attached incendiary bombs to balloons which travelled 5,000 miles toward the United States along the jet stream. The intention was to have the devices explode over the forested regions of the Pacific Northwest and start large forest fires that would divert precious U.S. manpower.
Geologist and historian J. David Rogers explains how they worked:
The balloons were crafted from mulberry paper, glued together with potato flour and filled with expansive hydrogen. They were 33 feet in diameter and could lift approximately 1,000 pounds, but the deadly portion of their cargo was a 33-lb anti-personnel fragmentation bomb, attached to a 64–foot long fuse that was intended to burn for 82 minutes before detonating. The Japanese programmed the balloons to release hydrogen if they ascended to over 38,000 feet and to drop pairs of sand filled ballast bags if the balloon dropped below 30,000 feet, using an onboard altimeter. Three-dozen sand-filled ballast bags were hung from a 4-spoke aluminum wheel that was suspended beneath the balloon, along with the bomb. Each ballast bag weighed between 3 and 7 pounds. The bags were programmed to be released in pairs on opposing sides of the wheel so the balloon would not be tipped to one side or another, releasing any of the precious hydrogen. In this way the balloons would rise in the daylight heat each day of the crossing and fall each evening, till their ballast bags were depleted, at which time the balloon and its deadly contents would descend upon whatever lay beneath it.
The first balloons were launched in late 1944, landing in the U.S. on November 5th off San Pedro, California. By the following day they landed as far away as Thermopolis, Wyoming. Some even landed in Canada. In all, some 285 confirmed landings and/or sightings were made. On March 5, 1945, six Americans (a minister and five children) were killed by one of the grounded balloons in Oregon while attempting to pull it through the forest back to their camp.
The U.S. government muzzled the media about the balloons for fear of encouraging the enemy. The American public was eventually made aware of the balloons after the war.