RAF Avro LancasterAvro Lancaster heavy bomber model. Entering service at the beginning of 1942, the Lancasters design grew out of a failed predecessor, the Avro Manchester. While its airframe offered a stable platform for heavy bombing assignments, the Manchesters twin engine design was inadequate to the task. By upgrading to four Merlins, the resulting aircraft met the nations needs and 7,366 Avro Lancasters were built during the war, the most of any British bomber.