B-29s, flying over Japan, dropped napalm-filled cluster bombs known as the M-69. Stewart Halsey Ross - in his book, Strategic Bombing by the United States in World War II: The Myths and the Facts - describes this devastating weapon, as follows:
...the M-69, [was] a 6.2 pound napalm bomblet of sophisticated design. Napalm was the remarkable material that had been developed by DuPont expressly as a filler for incendiary bombs, as a substitute for commonly used magnesium, the lightweight metal increasingly used in aircraft components.
The name was derived from its chemical compound, a mixture of naphthene and palmitic acids. This new compound when mixed with gasoline produced a thick, sticky, jelly-like material.
Napalm would revolutionize air-dropped incendiary weapons and would be used with equally devastating effect in flamethrowers by U.S. Marines in the Pacific war against Japanese troops.
By 1945, an enhanced formula, Napalm B, had been introduced; chemists added polystyrene and benzene to the lethal brew, yielding a longer burning fire at temperatures up to 2,000 degrees Fahrenheit, and even greater stickiness.
In external appearance the M-69 was deceptively innocent looking, disguising its sophisticated destructive capabilities inside a 3-inch-diameter, 20-inch-long plain steel cylinder. There were no stabilizing tail fins to make it look like a bomb; instead it contained a 3-foot-long strip of cloth, like a kite's tail, that popped out when it deployed to prevent tumbling.
As the sturdy unit punched its way through the thin roof of a house, a time-delay fuse was activated which, after 3 to 5 seconds, detonated an ejection-ignition charge. By this time, the bomblet would be at rest, on its side or its nose embedded in a floor.
The exploding charge ignited a small quantity of white-phosphorous powder which instantly set fire to the napalm and at the same time blew a fiercely burning glob of gasoline gel out of the tail of the casing. The burning gel could be propelled as far out as 100 feet. Whatever the glob hit, it stuck to; if the material was combustible, it immediately started an intense, hard-to-extinguish fire.
M-69 bombs were bound together in clusters of 38 and fitted inside a finned thin-walled container that opened clamshell-like over the target, scattering the bomblets. A primacord charge ran through the container's flange that was time-fuzed to explode and open the cluster at a predetermined altitude, generally about 2000 feet, an altitude that tests had shown yielded optimum dispersion.
A simpler, lighter construction was merely to strap the bombs together and attach a tail fin. The primacord cut the straps at fuze ignition, allowing the individual bombs to freely disperse.
Such configurations were not as aerodynamically stable as the "containerized" bomblets, but incendiary bombs were large-area weapons and were notoriously inaccurate.
Depending on aircraft speed and winds close to the ground, the bombs would be spread out over a keyhold-shaped area about 500 feet wide at its widest by about a half-mile long. A single B-29 on a firebomb raid over Japan typically would carry 40 clusters, or about 1,500 individual M-69s.
Quoted passage from Strategic Bombing by the United States in World War II: The Myths and the Facts, by Stewart Halsey Ross, page 108.