On the night of March 9-10, 1945, 339 B29 Superfortresses bombed Tokyo with phosphorous filled incendiary bombs. The intent was to burn the largely wooden homes and klll as many civilians as possible.

Dropping incendiary bombs on civilians began in WWI with the Germans droppng kerosene filled tanks on British cities. It was perfected in WWII by the British and applied by the American airforces over Germany for several years. A fire storm was created in Hamburg that destroyed that city and incendiary raids became quite common. A fire storm occured when the flames joined together, sucking in air and creating tornadic winds that simply consumed everything. The most famous such raid was against the medieval city of Dresden in February, 1945. American and Birtish experts estimated that 25,000 civilians were killed in the raid, but German and Russian experts estimated the deaths at 200,000, largely for political reasons.  There is no plausible military reason for this purely gratuitous raid.

The bombs used on these raids were very different from the 500lb to 4000lb high explosive bombs that were used against military targets. Incendiary bombs were small, the British version at 4 pounds, and contained various flammable substance like thermite or white phosphorous. The bombs were designed to breakthrough the roof of a house and, ideally, lodge in the attic, starting an intense and highly destructive fire. Incendiaries were dropped by the thousands so the area destroyed could be quite large.  Defenders developed the habit. of keeping wet blankets in attics that oculd be used to extinguish the bombs, so a delayed explosion bomb was developed that would explode 4 minutes late, killing the defenders.

The firebombing of Tokyo killed 100,000 civilians (and 65 other Japanese cities that killed another 500,000) and was the bloodiest night in history. This includes the bombing of Horoshima. The Japanese, after Hiroshima, commented that there had been many cities that had taken such a beating so there was no rush to surrender. The Russian invasion of Japan’s northern most island is what pushed the Emperor to surrender.. The fear of B29s was not equal to the fear of Russian boots in Tokyo, but the fear of B29s was very real as well.

This link is to a BBC  article supplies more information. http://www.bbc.com/news/world-asia-31809257