Fu-Go balloon bomb

The balloon bomb concept was a brainchild of the Imperial Japanese Army's Number Nine Research Laboratory (also known as the Noborito Laboratory), founded in 1927.[2] In 1933, Lieutenant General Reikichi Tada began an experimental balloon bomb program at Noborito, designated Fu-Go,[lower-alpha 1] which proposed a hydrogen balloon 13 feet (4.0 m) in diameter equipped with a time fuse and capable of delivering bombs up to 70 miles (110 km). The project was stopped by 1935 and never completed.[4]

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.[5] In summer 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 that could be launched from submarines.[6] On September 9, 1942, the latter was tested in the Lookout Air Raid, in which a Yokosuka E14Y seaplane was launched from a submarine off the Oregon coast. Warrant Officer Nobuo Fujita dropped two large incendiary bombs in Siskiyou National Forest in the hopes of starting a forest fire and safely returned to the submarine; however, response crews spotted the plane and contained the small blazes. The program was cancelled by the Navy.[7]

Also in September 1942, Major General Sueki Kusaba, who had served in the original balloon bomb program under Tada in the 1930s, was assigned to the laboratory and revived the Fu-Go project with a focus on longer flights.[7] 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.[8] According to U.S. interviews with Japanese officials after the war, the campaign was undertaken "almost exclusively for home propaganda purposes", with the Army having little expectations for its effectiveness.[9]

By March 1943, Kusaba's team developed a 20-foot (6.1 m) design capable of flying at 25,000 feet (7,600 m) for more than 30 hours.[10] The balloons were constructed from four to five thin layers of washi, a durable paper derived from the paper mulberry (kōzo) bush, which were glued together with konnyaku (Japanese potato) paste. The Army mobilized thousands of teenage girls at high schools across the country to laminate and glue the sheets together, with final assembly and inflation tests at large indoor arenas including the Nichigeki Music Hall and Ryōgoku Kokugikan sumo hall in Tokyo.[11] 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. A calibrated timer would release a 11-pound (5.0 kg) incendiary bomb at the end of the flight.[12] Two submarines (I-34 and I-35) were prepared and two hundred balloons were produced by August 1943, but attack missions were postponed due to the need for submarines as weapons and food transports.[10]

Engineers next investigated the feasibility of balloon launches against the United States from the Japanese mainland, a distance of at least 6,000 miles (9,700 km).[11] 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. The currents had been investigated by Japanese scientist Wasaburo Oishi in the 1920s; 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. Arakawa further found that the strongest winds blew from November to March at speeds approaching 200 miles per hour (320 km/h).[13]

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  Fu-Go carriage, with labeled ring, electrical circuits, fuses, ballast, and bombs
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  Top view of carriage assembly, with control device removed
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  Altitude control device, with central master aneroid barometer and backups
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  Bottom view of carriage fuses
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  Reconstructed balloon at the moment a blowout plug is detonated

Changing pressure levels in a fixed-volume balloon posed technical challenges. During the day, heat from the sun increased pressure, risking the balloon rising above the air currents or bursting. A relief valve was added to allow gas to escape when the envelope's internal pressure rose above a set level. At night, cool temperatures risked the balloon falling below the currents, an issue that worsened as gas was released. To resolve this, engineers developed a sophisticated ballast system with 32 sandbags mounted around a cast aluminum wheel, with each sandbag connected to gunpowder blowout plugs. The plugs were connected to three redundant aneroid barometers calibrated for an altitude between 25,000 and 27,000 feet (7,600 and 8,200 m), below which one sandbag was released; the next plug was armed two minutes after the previous plug was blown. A separate altimeter set between 13,000 and 20,000 feet (4,000 and 6,100 m) controlled the later release of the bombs. A one-hour activating fuse for the altimeters was ignited at launch, allowing the balloon time to ascend above these two thresholds. Tests of the design in August 1944 indicated success, with several balloons releasing radiosonde signals for up to 80 hours (the maximum time allowed by the batteries). A self-destruct system was added; a three-minute fuse triggered by the release of the last bomb would detonate a block of picric acid and destroy the carriage, followed by an 82-minute fuse that would ignite the hydrogen and destroy the envelope.[14]

B-Type rubberized silk balloon, with outline of a human for scale

In late 1942, the Imperial General Headquarters had directed the Navy to begin its own balloon bomb program in parallel with the Army project. Lieutenant Commander Kiyoshi Tanaka headed an effort that developed a 30-foot (9.1 m) rubberized silk balloon, designated the B-Type (in contrast to the Army's A-Type). The silk material was an effort to create a flexible envelope that could withstand pressure changes. The design was tested in August 1944, but the balloons burst immediately after reaching altitude, determined to be the result of faulty rubberized seams. The Navy program was subsequently consolidated under Army control, in part due to the declining availability of rubber as the war continued.[15] The B-Type balloons were later equipped with a version of the A-Type's ballast system and tested on November 2, 1944; one of these balloons, which was not loaded with bombs, became the first to be recovered by Americans after being spotted in the water off San Pedro, California, on November 4.[16]

The final balloon design was 33 feet (10 m) in diameter, and had a gas volume of 19,000 cubic feet (540 m³) and a lifting capacity of 300 pounds (140 kg) at operating altitude.[17] The bombs most commonly carried were:

• four 11-pound (5.0 kg) thermite incendiary devices, consisting of a steel tube 16 inches (41 cm) long and 4 inches (10 cm) in diameter with a mechanical impact fuse;
• one Type 92 33-pound (15 kg) high-explosive anti-personnel bomb, containing a block of picric acid or TNT surrounded by shrapnel rings;
• or alternatively to the anti-personnel bomb, one Type 97 26-pound (12 kg) incendiary bomb, containing three magnesium containers of thermite.[18]

Fu-Go launching sites and flight-following stations on Honshu

A balloon launch organization of three battalions was formed. The first battalion included headquarters and three squadrons totaling 1,500 men in Ibaraki Prefecture with nine launch stations at Ōtsu. The second battalion of 700 men in three squadrons operated six launch stations at Ichinomiya, Chiba; and the third battalion of 600 men in two squadrons operated six launch stations at Nakoso, Fukushima. The Ōtsu site featured its own hydrogen plant, while the second and third battalions used hydrogen gas manufactured at factories near Tokyo. The combined launching capacity of the sites was about 200 balloons per day, with 15,000 launches planned through March.[19] The Army estimated that 10 percent of the balloons would survive the journey across the Pacific Ocean.[8]

Each launch pad consisted of anchor screws drilled into the ground and arranged in a circle the same diameter as the balloons. After laying out a deflated envelope, hoses were used to fill the envelope with hydrogen before it was tied down with guide ropes and detached from the anchors. The carriage was attached and the guide ropes were disconnected. Each launch took between thirty minutes and an hour, depending on the presence of surface winds that made releases difficult.[20] The best time to launch was just after the passing of a high-pressure front, and wind conditions were most suitable for several hours prior to the onshore breezes at sunrise. Suitable launch conditions were expected for only about fifty days through the winter period of maximum jet stream velocity.[19]

The first balloons were launched at 0500 on November 3, 1944. Some balloons in each of the launches carried radiosonde equipment instead of bombs, and were tracked by direction finding stations in Ichinomiya, at Iwanuma, Miyagi, at Misawa, Aomori, and on Sakhalin to estimate the progress of the balloons towards North America.[21]

Two weeks after the discovery of the B-Type balloon off San Pedro, an A-Type balloon was found in the ocean off Kailua, Hawaii, on November 14. More appeared near Thermopolis, Wyoming, on December 6 (with an explosion heard by witnesses, and a crater later located) and near Kalispell, Montana, on December 11, followed by finds near Marshall, Alaska, and Estacada, Oregon, later in the month. National and state agencies were placed on heightened alert, and forest rangers were asked to report sightings or finds. The balloons continued to be discovered across North America on a near daily basis, with sightings and partial or full recoveries in Alaska, Arizona, California, Colorado, Idaho, Iowa, Kansas, Michigan (where the easternmost of the balloons was found at Farmington), Montana, Nebraska, Nevada, North Dakota, Oregon, South Dakota, Texas, Utah, Washington, and Wyoming; as well as in Canada in Alberta, British Columbia, Manitoba, Saskatchewan, and the Northwest and Yukon Territories; in northwestern Mexico; and at sea by passing ships.[lower-alpha 2][23]

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  Balloon found near Alturas, California, on January 10, 1945, reinflated for tests
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  Balloon found near Bigelow, Kansas, on February 23, 1945
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  Balloon found near Nixon, Nevada, on March 29, 1945
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  Aerial photograph of a balloon taken from an American plane
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  Aerial photograph of a balloon taken from an American plane

American authorities concluded the greatest danger from the balloons would be wildfires in the coastal forests of the Pacific Northwest during dry months. The Fourth Air Force, Western Defense Command, and Ninth Service Command organized the "Firefly Project" with a number of Stinson L-5 Sentinel and Douglas C-47 Skytrain aircraft and 2,700 troops, including 200 paratroopers of the 555th Parachute Infantry Battalion, who were stationed at critical points for use in firefighting missions.[24] Through Firefly, the military used the United States Forest Service as a proxy, unifying fire suppression communications among federal and state agencies and modernizing the Forest Service through the influx of military personnel, equipment, and tactics.[25] In the "Lightning Project", health and agricultural officers, veterinarians, and 4-H clubs were instructed to report any strange new diseases of crops or livestock caused by potential biological warfare. Stocks of decontamination chemicals, ultimately unused, were shipped to key points in the western states.[24] A report by U.S. investigators, based on interviews with Imperial Army officials after the war, concluded that there had been no plans for chemical or biological payloads.[26]

Balloon shot down by P-38 Lightning fighters at Attu, Aleutian Islands, Alaska, on April 13, 1945

Army Air Forces and Navy fighters were scrambled on several occasions to intercept balloons, but they had little success due to inaccurate sighting reports, bad weather, and the high altitude at which the balloons traveled.[24] In all, about 20 of the balloons were shot down by aircraft.[lower-alpha 3][27] Experiments conducted on recovered balloons to determine their radar reflectivity also had little success. In the "Sunset Project" initiated in early April 1945, the Fourth Air Force attempted to detect the radio transmissions emitted by tracking balloons using sites in coastal Washington; 95 suspected signals were detected, but were of little use for interception due to the relatively low percentage of balloons with transmitters, and observed fading of the signals as they approached the coast.[24]

Few American officials believed at first that the balloons could have come directly from Japan. Early U.S. theories speculated that they were launched from German prisoner of war camps or from Japanese-American internment centers. After bombs of Japanese origin were found, it was believed that the balloons were launched from coastal submarines.[28] Statistical analysis of valve serial numbers suggested that tens of thousands of balloons had been produced.[9] Sand from the sandbags was studied by the Military Geology Unit of the United States Geological Survey, revealing mineral and diatom compositions that corresponded to Ichinomiya. Aerial reconnaissance later located two nearby hydrogen production facilities, which were destroyed by B-29 bombing raids in April 1945.[29]

Map of Fu-Go incident locations in North America. Sites marked with a black dot.

By mid-April 1945, Japan lacked the resources to continue manufacturing balloons, with both paper and hydrogen in short supply. Furthermore, the Army had little evidence that the balloons were reaching North America, let alone causing damage. The campaign was halted, with no intention to revive it when winds restarted in late 1945.[38] In total, about 9,300 balloons were launched in the campaign (approximately 700 in November 1944, 1,200 in December, 2,000 in January 1945, 2,500 in February, 2,500 in March, and 400 in April), of which about 300 were found or observed in North America.[39] The Fu-Go balloon was the first weapon system to have intercontinental range, with its flights being the longest-ranged attacks in the history of warfare at the time.[1]

No wildfires were positively identified as being caused by balloon bombs.[40] As predicted by Imperial Army officials, the winter and spring launch dates had limited the chances of the incendiary bombs starting forest fires due to the high levels of precipitation in the Pacific Northwest; forests were generally snow-covered or too damp to catch fire easily.[41] Furthermore, much of the western U.S. received disproportionately more precipitation in 1945 than in any other year in the decade, with some areas receiving 4 to 10 inches (10 to 25 cm) of precipitation more than normal.[25] Many of the recovered balloons also had a high percentage of unexploded plugs, caused by failure of their batteries or fuses.[24] The most tactically successful attack took place on March 10, 1945, when one of the balloons descended near Toppenish, Washington, colliding with power lines and causing a short circuit that cut off power to the Manhattan Project's production facility at the state's Hanford Engineer Works. Backup devices restored power to the site, but it took three days for its nuclear reactors to be brought to full capacity; the plutonium produced in the reactors was later used in Fat Man, the atomic bomb dropped on Nagasaki in August 1945.[42]

Mitchell Monument in 2011

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.

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. While Archie was moving the car, Elsie and the children found the balloon and carriage, loaded with an anti-personnel bomb, on the ground. A large explosion occurred; the four boys (Edward Engen, 13; Jay Gifford, 13; Dick Patzke, 14; and Sherman Shoemaker, 11) were killed instantly, while Joan Patzke (13) and Elsie died shortly afterwards.[43] A bomb disposal expert guessed that the bomb had been kicked or otherwise disturbed. Military personnel who arrived on the scene observed that the balloon had snow beneath it, unlike the surrounding area, and concluded that it had lain there undisturbed for weeks until discovered. The U.S. press blackout was lifted on May 22 so the public could be warned of the balloon threat.[44]

A memorial, the Mitchell Monument, was built in 1950 at the point of the explosion.[45] The surrounding Mitchell Recreation Area was listed on the National Register of Historic Places in 2003.[46] A nearby ponderosa pine still bears scars on its trunk from the bomb's shrapnel. In 1987, a group of Japanese women who were involved in Fu-Go production as schoolgirls delivered 1,000 paper cranes to the families of the victims as a symbol of peace and forgiveness, and cherry trees were planted around the monument on the fiftieth anniversary of the incident in 1995.[47]

The remains of balloons have continued to be discovered after the war. At least eight were found in the 1940s, three in the 1950s, two in the 1960s, and one in the 1970s.[48] A carriage with a live bomb was found near Lumby, British Columbia, in 2014 and detonated by a Royal Canadian Navy ordnance disposal team.[49] Remains of another balloon were found near McBride, British Columbia, in 2019.[50] Many war museums in the U.S. and Canada exhibit Fu-Go fragments, including the National Air and Space Museum and Canadian War Museum.[51]

• Operation Outward – WWII British attack on Germany with balloon bombs
• E77 balloon bomb – 1950s U.S. biological weapon inspired by Fu-Go
• WS-124A Flying Cloud – 1950s U.S. Air Force balloon bomb program
• 2023 Chinese balloon incident – Incursion of U.S. airspace by an alleged spy balloon

• Conley, Cornelius W. "The Great Japanese Balloon Offensive". Air University Review. February–March 1968: 68–83.
• Cook, Haruko Taya; Cook, Theodore F., eds. (1992). Japan at War: An Oral History. New York: New Press. ISBN 978-1-56584-014-0.
• McPhee, John (January 29, 1996). "Balloons of War". The New Yorker. pp. 52–60.
• Webber, Bert (1975). Retaliation: Japanese Attacks and Allied Countermeasures on the Pacific Coast in World War II. Corvallis: Oregon State University Press. ISBN 978-0-87071-076-6.

Wikimedia Commons has media related to Fu-Go fire balloons (World War II).

• Report by U.S. Technical Air Intelligence Center, May 1945
• The short film Japanese Paper Balloon is available for free download at the Internet Archive.