On the night of Apr 13, 1945*, 550 B-29s struck Tokyo in the biggest fire bombing of the war to date.
About 1145pm on that spring Friday night, an incendiary bomblet struck the roof of the Kodokan just to the east of the seat of Kano shihan in the Dai Dōjō (‘Great Dōjō’, the largest of several in the building). It was probably an M65, a 6 pound (2.7 kg) napalm bomblet released from a 500 pound (228 kg) cluster bomb, one of scores dropped by a B29 Superfortress heavy bomber from about 2500 ft altitude, and the bomblet probably penetrated partway through the roof.
The previous high altitude, conventional bombing campaign against Japan had been considered ineffective, and the bombers were ordered to fly at very low altitudes to dispense their new fire bombs, despite much heavier losses to Japanese antiaircraft fire and interceptor fighter attacks than the previous high altitude attacks.

1 to 3 seconds after impact with the roof, the bomblet fuzed a phosphorous bursting charge that splattered flaming jellied gasoline napalm over the Kodokan roof.
Some of the burning jellied gasoline napalm burned through the roof, setting fire to the large, elaborate 神棚 kamidana Shintō ‘god shelf’ that Kano shihan had installed on the front wall of the Dai Dōjō. Fire reached the tatami below, burning around 70 mats; soon the fire outside the building burned together with the fire inside to threaten the entire structure.
A fire truck team arrived but low water pressure hampered their efforts. Then, the night watch and local couples cheered on the local tonari gumi neighborhood association, which used Kodokan hoses and buckets to assist the fire truck, putting the Kodokan fire out, but much of the neighborhood was destroyed. Almost 3500 people died and over 170,000 buildings were destroyed that single night raid.

a huge postwar study of the effectiveness of the bombing campaign against Japan.
Kodokan head Rear Admiral Nangō Jirō, Kano shihan’s nephew, was determined to renew training. He housed local refugees whose homes had been burned down in the undamaged portion of the Dai Dōjō, but had the walls of the rooms near the third floor research dōjō knocked down to enlarge it; regular practice resumed on the 16th.

the second Kodokan kanchō (institute head) and Kanō’s nephew
(Photo above from Wiki Commons, probably taken from a Japanese newspaper interview after the death of his second son in World War II. Story to be related later.)
The Dai Dōjō was repaired by the end of 1945 and was used for the training 寒稽古 kangeiko ‘cold weather training’ traditional in the new year of 1946.
NOTES:
* 全日本柔道連盟 Zen Nihon Jūdō Renmei All Japan Judo Federation history cites a date that does not match US Army Air Corps records of the raid dates; this history uses US military record dates when available. http://www.judo.or.jp )
Lance Gatling
Author / Lecturer
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