||The Avro Lancaster was the most successful and well-known of the British heavy bombers of the Second World War. A refinement of the earlier Avro Manchester, the first Lancaster flew in January 1941. Lancasters began equipping Royal Air Force (RAF) squadrons in December of that year and these flew their first operational sorties in March 1942. Lancaster production began in Canada in 1942 and the first of the Canadian-made variants were delivered to Britain in August 1943. In the end, 7,378 Lancasters were produced. It was intended to begin Lancaster production in Australia in 1944 and a "pattern" aircraft - "Queenie VI" - arrived in Australia in mid-1943. Production, however, never eventuated, and "Queenie VI" was used mainly to garner publicity for war loans drives, which included a flight under the Sydney Harbour Bridge.
The Lancaster proved the most versatile of the British heavy bombers. Without modification it could carry loads of up to 8,165 kg, or if converted could carry both 12,000 lbs [5,443 kg] "Tallboy" bombs, and 22,000 lbs [9,979 kg] Grand Slam deep penetration bombs - the only bomber to carry bombs of this size in the Second World War. From 1942 the Lancaster became Bomber Command's most important bomber with 61 squadrons eventually being equipped with the aircraft. Although robust, the Lancaster's critical vulnerability was its defensive armament. At a time when most fighter aircraft were armed with .50-in calibre machine-guns and small calibre cannons firing explosive shells, the Lancaster with its .303-in calibre machine guns was exceedingly lightly armed. In addition there were no guns or even observation points covering the Lancaster's underside - a feature exploited by German night fighters with upward firing cannon.