The Blitz
Based on experience with German strategic bombing during World War I against the United Kingdom, the British government estimated after the war that 50 casualties— with about one third killed— would result for every tonne of bombs dropped on London. The estimate of tonnes of bombs an enemy could drop per day grew as aircraft technology advanced, from 75 in 1922, to 150 in 1934, to 644 in 1937. That year the Committee on Imperial Defence estimated that an attack of 60 days would result in 600,000 dead and 1,200,000 wounded. News reports of the Spanish Civil War, such as the bombing of Barcelona, supported the 50-casualties-per-tonne estimate. By 1938 experts generally expected that Germany would attempt to drop as much as 3,500 tonnes in the first 24 hours of war and average 700 tonnes a day for several weeks. In addition to high explosive and incendiary bombs the enemy would possibly use poison gas and even bacteriological warfare, all with a high degree of accuracy. In 1939 military theorist Basil Liddell-Hart predicted that 250,000 deaths and injuries in Britain could occur in the first week of war.

Q: How many tons did experts expect Germany to drop in the first 24 hours of the war?
3,500