Heavy investment had been made in building up the bomber force, but faith in its potential was beginning to wane, and Harris realised a major propaganda success was vital to demonstrate his belief that bombers could be decisive in defeating the enemy. Harris began pushing for a mass raid using the magic number of 1,000 bombers, although in fact the RAF barely had that many. At last, using every plane available including trainee crews, the RAF raided Cologne on 30/31 May 1942 with over 1,000 bombers; although over half the city was destroyed and it was seen as a success, the city made a surprising recovery. RAF assaults on medium-sized industrial towns to the east of the Rhine, the Ruhr and Berlin from mid-1942 also did little to weaken Germany economically. From July the B-24 Liberator and Flying Fortress fleets of the United States Air Force (USAAF) took on the role of daytime precision bombing of German arms and communication targets. They began by raiding airfields and railway stations in France and the Netherlands and badly damaged the Heroya aluminium centre near Trondheim in Norway which produced synthetic cryolite, used in the manufacture of aluminium. From mid-November the RAF began a series of 16 massed night raids on Berlin, but though the damage was considerable, the raids were less effective than those on the Ruhr and Hamburg. Essen and Bremen also suffered 1,000 plane raids and upwards of 1,000 tons of bombs. In 1942 the RAF dropped 37,000 tons of bombs on German targets, probably three times the weight dropped on Britain in 1940 and early 1941.