Under the leadership of Arthur Harris the British bomber force grew in size.[13] British scientific advances produced navigation aids which improved on bombing accuracy after the controversy of the Butt Report.[14][lower-alpha 2] British industries were also producing heavy bombers which replaced many of the elderly pre-war medium bomber designs.[lower-alpha 3] The consequences of these developments in the air war and Harris' determination to end the war through bombing, were a series of destructive attacks on German industrial cities.