Between 7 September 1940 and 21 May 1941, 16 British cities suffered aerial raids with at least 100 long tons of high explosives. Over a period of 267 days, London was attacked 71 times, Birmingham, Liverpool and Plymouth eight times, Bristol six, Glasgow five, Southampton four, Portsmouth and Hull three and a minimum of one large raid on eight other cities. This was a result of a rapid escalation starting on 24 August 1940, when night bombers aiming for RAF airfields drifted off course and accidentally destroyed several London homes, killing civilians, combined with the UK Prime Minister Winston Churchill's retaliatory bombing of Berlin on the following night.[clarification needed] 

From 7 September 1940, one year into the war, London was bombed by the Luftwaffe for 57 consecutive nights. More than one million London houses were destroyed or damaged and more than 40,000 civilians were killed, almost half of them in London. Ports and industrial centres outside London were also attacked. The main Atlantic sea port of Liverpool was bombed, causing nearly 4,000 deaths within the Merseyside area during the war. The North Sea port of Hull, a convenient and easily found target or secondary target for bombers unable to locate their primary targets, was subjected to 86 raids in the Hull Blitz during the war, with a conservative estimate of 1,200 civilians killed and 95 percent of its housing stock destroyed or damaged. Other ports including Bristol, Cardiff, Portsmouth, Plymouth, Southampton and Swansea were also bombed, as were the industrial cities of Birmingham, Belfast, Coventry, Glasgow, Manchester and Sheffield. Birmingham and Coventry were chosen because of the Spitfire and tank factories in Birmingham and the many munitions factories in Coventry. The city centre of Coventry was almost destroyed, as was Coventry Cathedral.
What were the fighters aiming at?
 for RAF airfields