The uncontrolled balloons were carried over the Pacific Ocean from Japan to North America by fast, high-altitude air currents, today known as the jet stream, and used a sophisticated sandbag ballast system to maintain their altitude.

Between November 1944 and April 1945, the Imperial Japanese Army launched about 9,300 balloons from sites on coastal Honshu, of which about 300 were found or observed in the U.S., Canada, and Mexico.

[3] After the Doolittle Raid in April 1942, in which American planes bombed the Japanese mainland, the Imperial General Headquarters directed Noborito to develop a retaliatory bombing capability against the U.S.[4] In mid-1942, Noborito investigated several proposals, including long-range bombers that could make one-way sorties from Japan to cities on the U.S. West Coast, and small bomb-laden seaplanes which could be launched from submarines.

[6] Also in September 1942, Major General Sueki Kusaba, who had served under Tada in the original balloon bomb program in the 1930s, was assigned to the laboratory and revived the Fu-Go project with a focus on longer flights.

[6] The Oregon air raid, while not achieving its strategic objective, had demonstrated the potential of using unmanned balloons at a low cost to ignite large-scale forest fires.

[7] According to U.S. interviews with Japanese officials after the war, the balloon bomb campaign was undertaken "almost exclusively for home propaganda purposes", and the Army had little expectation of its effectiveness.

[10] The original proposal called for night launches from submarines located 600 miles (970 km) off of the U.S. coast, a distance the balloons could cover in 10 hours.

[10] Engineers sought to make use of strong seasonal air currents discovered flowing from west to east at high altitude and speed over Japan, today known as the jet stream.

In late 1943, the Army consulted Hidetoshi Arakawa of the Central Meteorological Observatory, who used Oishi's data to extrapolate the air currents across the Pacific Ocean and estimate that a balloon released in winter and that maintained an altitude of 30,000 to 35,000 feet (9,100 to 10,700 m) could reach the North American continent in 30 to 100 hours.

Lieutenant Commander Kiyoshi Tanaka headed a group which developed a 30-foot (9.1 m) rubberized silk balloon, designated the B-Type (in contrast to the Army's A-Type).

The balloons continued to be discovered across North America, with sightings and partial or full recoveries in Alaska, Arizona, California, Colorado, Hawaii, Idaho, Iowa, Kansas, Michigan (where an incendiary bomb was found at Farmington in the easternmost incident), Montana, Nebraska, Nevada, North Dakota, Oregon, South Dakota, Texas, Utah, Washington, and Wyoming; in Canada in Alberta, British Columbia, Manitoba, Saskatchewan, and the Northwest and Yukon Territories; in Mexico in Baja California Norte and Sonora; and at sea.

In response to the threat of wildfires in the Pacific Northwest during the summer months, the Army's Western Defense Command (WDC), Fourth Air Force, and Ninth Service Command organized the "Firefly Project" with Stinson L-5 Sentinel and Douglas C-47 Skytrain aircraft and 2,700 troops, including 200 paratroopers from the all-black 555th Parachute Infantry Battalion, who were deployed in 36 firefighting missions between May and October 1945.

[29] In the WDC's "Lightning Project", health and agricultural officers, veterinarians, and 4-H clubs were instructed to report any new diseases of crops or livestock caused by potential biological warfare.

[31]Army Air Forces and Navy fighter planes were scrambled on several occasions to intercept balloons, but they had little success due to inaccurate sighting reports, bad weather, and the very high altitude at which they traveled.

[34] On January 4, 1945, the U.S. Office of Censorship sent a confidential memo to newspaper editors and radio broadcasters asking that they give no publicity to balloon incidents; this proved highly effective, with the agency sending another memo three months later stating that cooperation had been "excellent" and that "there is no question that your refusal to publish or broadcast information about these balloons has baffled the Japanese, annoyed and hindered them, and has been an important contribution to security.

[37] Starting in mid-February 1945, Japanese propaganda broadcasts falsely announced numerous fires and a panicked American public, further claiming casualties in the hundreds or thousands.

[41] In late April, censors investigated the nationally-syndicated comic strip Tim Tyler's Luck by Lyman Young, which depicted a Japanese balloon recovered by the crew of an U.S. submarine.

In subsequent weeks, its protagonists fought monster vines which sprang from seeds the balloon was carrying, created by an evil Japanese horticulturalist.

A few weeks later, the comic strip Smilin' Jack by Zack Mosley depicted a plane crashing into a Japanese balloon, which exploded and started a fire upon falling to the ground.

[29] The most damaging attack occurred on March 10, 1945, when a balloon descended near Toppenish, Washington, and collided with electric transmission lines, causing a short circuit which cut off power to the Manhattan Project's production facility at the state's Hanford Engineer Works.

[49] On May 5, 1945, six civilians were killed near Bly, Oregon, when they discovered one of the balloon bombs in Fremont National Forest, becoming the only fatalities from Axis action in the continental U.S. during the war.

[50][51] Reverend Archie Mitchell and his pregnant wife Elsie (age 26) drove up Gearhart Mountain that day with five of their Sunday school students for a picnic.

In 1987, a group of Japanese women involved in Fu-Go production as schoolgirls delivered 1,000 paper cranes to the victims' families as a symbol of peace and healing, and six cherry trees were planted at the site on the incident's 50th anniversary in 1995.

Thus, a single interview with Lieutenant Colonel Terato Kunitake of the Army General Staff and a Major Inouye became the source of nearly all information on the project's objectives for U.S. investigators.

A five-volume report, prepared by a team led by Karl T. Compton of the Office of Scientific Research and Development and Edward L. Moreland of MIT, was later submitted to President Harry S.

[58] A carriage with a live bomb was found near Lumby, British Columbia, in 2014 and detonated by a Royal Canadian Navy ordnance disposal team.