The M-69 incendiary cluster bomb was used to bomb Japanese cities during World War II. They were nicknamed "Tokyo calling cards". The M-69 was a plain steel pipe with a hexagonal cross section 3 inches (7.6 cm) in diameter and 20 inches (51 cm) long. It weighed about 6 pounds (2.7 kg).
The bomb used napalm (jelled gasoline) as an incendiary filler, improving on earlier designs which used thermite or magnesium fillers that burned more intensely but were less energy and weight efficient and were easier to put out. In Germany they were filled with jellied oil and dropped in clusters of 36 in the non-aerodynamic M-19 bomb. Over Japan they were used in clusters of 38 as part of the finned E-46 'aimable cluster', which opened up at about 2,000 ft (610 m). After separation, each of the 38 M-69s would release a 3-foot (1 m) cotton streamer to orient its fuze downward. Upon hitting a building or the ground, the timing fuze burned for three to five seconds and then a white phosphorus charge ignited and propelled the incendiary filling up to 100 feet (30 m) in several flaming globs, instantly starting multiple intense fires.
It was tested against typical German and Japanese residential structures at Japanese Village and German Village, constructed at Dugway Proving Ground, Utah, in 1943. The M-69 was the most successful incendiary in the tests.
Against Japan, the M-69 was carried in the bomb bay of the Boeing B-29 Superfortress, with a typical load containing 40 cluster bombs, a total of 1520 M-69 bomblets. The bombs were very effective in setting fire to Japanese cities in mass firebombing raids starting in February 1945 against Kobe. In the first ten days of March 1945, raids with the M-69 and M-47, extensive damage was done to Tokyo, Nagoya, Osaka, and Kobe.
- 180 Degrees Out: The Change in U.S. Strategic Bombing Applications, 1935-1955- Dissertation of John M. Curatola, DPhil University of Kansas (2008). Quoting "Tokyo Calling Cards", Collier’s Magazine, April 1945, 44 and 58.
- Ross, Stewart Halsey (2002). Strategic Bombing by the United States in World War II: The Myths and the Facts. McFarland. pp. 107–108. ISBN 9780786414123.
- Science: Incendiary Jelly, Time, Apr. 02, 1945
- Sion, Edward M. (2008). Through Blue Skies to Hell: America's Bloody 100th in the Air War Over Germany. Casemate Publishers. p. 20. ISBN 9781935149965.
- Bradley, F.J. (1999). No Strategic Targets Left. Turner Publishing. p. 33. ISBN 9781563114830.
- World Battlefronts: BATTLE OF THE PACIFIC: Firebirds' Flight, Time, Mar. 19, 1945
- World Battlefronts: Ten-Day Wonder, Time, Mar. 26, 1945
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