They weren't.
The British night raids were largely ineffective due to bomb dispersal. Germans had a pretty well-running fire- and repair-service that put out the fires and got electricity, water and the mass transit running quickly again after a city was bombed. The US day raids were more accurate thanks to daylight and better bomb sights, but they also suffered heavy losses.
What was really effective was targeting the choke points of industry. U-boat battery plant has already been mentioned, the synthetic oil plants and the Ploesti oil fields were another effective target, the ball bearing plant in Schweinfurt was critical for production of tanks (ball bearings are necessary for creating a turret that can swivel 360 without limits) and the repeated damage caused to the plant was one of the reasons why Germany relied more and more on assault guns instead of tanks.
Douhet's idea in the inter-war period, and the similar thinking of other air war theorists was that the civilian population would be so demoralized by massive bombings that they would revolt against their government and thus end the war. Or they would flee the cities and factories and your enemy would have to surrender because they couldn't keep their war machine running. Both assumptions turned out to be completely false.
The best proof of this is, that despite the increased bombing raids in 1944, many of which were on a massive scale of near or over 1,000 bombers, the German production of both tanks and fighters actually increased. But that didn't really help them that much because their fuel situation was becoming critical.