After their defeat in the Battle of the Heligoland Bight in 1939, the British abandoned daylight raids in favour of night bombing. The appointment of Arthur Harris, Air Officer Commanding (AOC) Bomber Command changed the nature of the night war. Area bombardment became the tactical method of destroying a target after the damning Butt Report in 1941. The result of improved navigational aids and new heavy bomber designs, the British carried out destructive air attacks on German cities, beginning with the attack on Lübeck in March 1942, and then the bombing of Hamburg in July 1943. In June 1940 the "Luftwaffe" still had no proper night fighter training school. At the end of 1941, the OKL had time to build a night fighter force capable of imposing an intolerable cost on the RAF, but the OKL chose not to do so, perhaps under the illusion the Soviet Union would soon fall and air units then released to the West for air defence. The shock effect of Hamburg on the OKL encouraged greater urgency in producing new tactics and technologies to overcome the British offensive, now the Germans had lost the lead in high-frequency field. The Nazi leadership, in particular Speer and Joseph Goebbels were stunned by the impact which destroyed 40 percent of large firm and 80 percent of small firm production, along with 75 percent of the electrical works, 90 percent of the gas and 60 percent of water systems.