The four-engined heavy bomber of the RAF, the Avro Lancaster, performed a variety of bombing operations and other roles during WWII. The B.I (Special) configuration of the aircraft, known as the "Grand Slam Bomber" was modified to carry the largest bomb made in war, the 22,000lb (9,979kg) "Grand Slam bomb". The first time the Grand Slam bomb was used successfully was on March 14th of 1945, in the attack against the Bielefeld Viaduct in northern Germany. This viaduct was a vital railroad node connecting the Ruhr industrial zone with large cities such as Hanover. Despite past bombing of the viaduct by the RAF and US Air Force, it had not yet sustained any critical damage. On that day, 617 Squadron, an elite group known as the "dambusters", flew into action. Sqn. Ldr. C.C. Calder dropped a Grand Slam bomb which landed about 30m from the viaduct. The underground explosion sent out powerful shock waves, destroying the majority of the viaduct. On March 19th, 617 Squadron also destroyed the Arnsberg Viaduct. By the end of the war, a total of 41 Grand Slam bombs were dropped, dealing a heavy blow to Germany. 33 Lancasters were converted to Grand Slam Bombers including aircraft no.: PB592, PB995 through PB998, and PD112 through PD139.