Avro 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. Armament included eight to ten Browning machine guns for fighter defense (depending on model variant) mounted in the nose, upper dorsal turret and the tail. Experience with a variety of bomb loads eventually led to adoption of the Grand Slam 22,000-pound bomb, the largest carried by any aircraft in the war. For the dam-busting strike in May 1943, the Lancaster dropped British designer Barnes Walliss bouncing bombs which skipped on the surface before impact. Wartime Lancaster sorties totaled about 156,000 during which roughly 608,000 tons of ordnance were dropped on the enemy.