The Avro Lancaster is a British four-engined Second World War heavy bomber designed and built by Avro for the Royal Air Force (RAF). It first saw active service with RAF Bomber Command in 1942 and, as the strategic bombing offensive over Europe gathered momentum, it was the central implement for the night-time bombing campaigns that followed. It became the main heavy bomber used by the RAF, the RCAF, and squadrons from other Commonwealth and European countries serving within the RAF, overshadowing its close contemporaries the Handley Page Halifax and Short Stirling. The "Lanc", as it was affectionately known, thus became one of the more famous and most successful of the Second World War night bombers, "delivering 608,610 long tons of bombs in 156,000 sorties." The Lancaster, an evolution of the troublesome Avro Manchester, was designed by Roy Chadwick and was powered by four Rolls-Royce Merlins, or, in one version, Bristol Hercules engines.