In November, 1944, the Japanese began launching unmanned bomb-carrying balloons, which travelled on prevailing winds across the Pacific Ocean to North America. It was hoped that the balloons would start forest fires and cause general panic among the population.
The balloons measured about 33 feet (10 m) in diameter and 70 feet (21 m) from the top of the balloon to the payload at the bottom. They were first made of paraffined paper, and later from latex and fabricated silk, and contained hydrogen gas. The payload consisted of 36 sand-filled paper bags for use as ballast, 4 incendiary bombs and 1 33-pound anti-personnel bomb.
The balloons began their three to five day journey from Japan at an altitude of about 35,000 feet (10.7 km), usually travelling at speeds between 80 and 120 miles per hour (129 and 193 km per hour). As gas slowly leaked from the balloons, they descended in altitude. When they fell to about 25,000 feet (7.62 km), a barometric pressure switch would cause one of the ballast sandbags to be dropped, and the balloons would rise again to 35,000 feet. This up and down pattern continued as the balloons crossed the Pacific Ocean. When the balloons reached the west coast of North America, they were supposed to have exhausted their supply of ballast sandbags and the bombs would then be used as ballast, with one bomb being dropped with each descent to 25,000 feet as they travelled across land. After the final bomb was dropped, a fuse would be ignited and the balloons would destroy themselves in bright orange fireballs.
It is estimated that about 9,000 of the balloons were launched by Japan between November, 1944 and April, 1945, but it is believed that less than 1,000 of them actually reached North America, with most of the rest self-destructing over, or falling into, the sea. Of those that did reach land, some were seen exploding in the air and others were found on the ground in remote areas, usually with the bomb loads missing but occasionally with some bombs still attached. The balloons reached Alaska, Canada, Mexico and 16 U.S. states, travelling as far east as Michigan and Texas. Most of the balloons were sighted or found in British Columbia, Washington state, Oregon, California, and Montana. Several minor forest fires, in California and Oregon, were possibly caused by the balloons, but this was never completely verified.
The first discovery of a balloon in North America was made by two woodchoppers, who discovered a balloon on the ground near Kalispell, Montana in December, 1944. It was determined that the balloon originated in Japan by analyzing beach sand from one of the balloon's ballast sandbags. Tight censorship was immediately imposed on further balloon sightings, since it was feared that disclosing when and where balloons were being found would encourage the enemy to launch more balloons and perfect their delivery. It was also thought that the balloons posed little danger to the public at large, so even though some government and military officials and newspapermen knew about the bombing balloons early on, the general public was not told about them until May of 1945, about six months after they were first launched.
The public announcement was finally made due to a tragic event that occurred on May 5, 1945. A woman and five children, on a church picnic, were killed in a remote area near Bly, Oregon, after they discovered a downed balloon with a bomb still attached, and one of them moved the bomb, causing it to explode. These deaths were the only known fatalities on the U.S mainland from enemy attack during World War II. At first it was officially reported that the deaths were due to an explosion of an 'unknown object', but after much debate, it was finally decided that the public's need to be informed about the existence of the bombing balloons outweighed any military advantage that the enemy might gain from the disclosure. It was also feared that, with the end of the school year approaching, there would soon be many children exploring in remote areas where unexploded bombs were most likely to be found. So in late May, the general public was finally informed about the existence of the bombing balloons, although details about individual balloon encounters were still withheld, except for the one fatal incident mentioned above.
The period of censorship appeared to have served its purpose, since it was later learned that the Japanese scaled down and eventually abandoned the balloon launchings, considering them ineffective since they had heard of very few balloons reaching U.S. territory.
Many of the balloons had been made by patriotic Japanese school children as a part of the war effort. In 1987, several tried to atone. They folded 1,000 paper cranes, a Japanese symbol of healing and peace, and sent them to the families of the Oregon picnickers. Here is an excerpt from one of the accompanying letters:
"We participated in the building of weapons used to kill people without understanding much beyond the knowledge that America was our adversary in a war. To think that the weapons we made took your lives as you were out on a picnic! We were overwhelmed with deep sorrow."