In July 1940, the first production Stirling departed Rochester; in August 1940, it was delivered to No. 7 Squadron at RAF Leeming, North Yorkshire. Following a four-month working-up period in which crews adapted to operating the type, the Stirling attained operational status in January . On the night of 10/11 February 1941, the first operational combat mission was performed, flown by the first three Stirlings, against fuel storage tanks at Vlaardingen near Rotterdam, in the Netherlands, all but two bombers were deployed during the mission, which was considered to have run smoothly. By the end of 1941, more than 150 Stirlings had been completed and three RAF squadrons had been equipped with it. Stirlings flew on day and night bombing operations and had been found to be most capable of standing up to enemy interceptor aircraft by using a sweeping combination of fighters and bombers, a tactic which became known as the Circus offensive.