The four-engine Avro Type 683 Lancaster was developed from the disappointing twin-engine Manchester, which was unreliable due to engine fires. The wingspan was increased and the engines replaced with Merlins. The first flight was made in January 1941 and it soon proved its excellence in handling and high load capability and became the backbone of the RAF Bomber Command. The type was utilized at the M.A.N factory raid in Augsburg, the Dambusters Raid on the Ruhr Valley dams, and in sinking the Tirpitz. At the end of the war it was used in relief operations, dropping supplies to the Dutch and returning prisoners of war from Germany. Postwar they were used for testing jet engines and in-flight fueling systems. The last one was withdrawn from service in October 1956 although the RCAF and French Air Force still used them until the mid-1960s.