Danny: Further to your #11504, I believe it was the introduction of the Air Bomber category circa mid-1942 that permitted the change to single-pilot operation in Bomber Command. There's lots about the progressive change in aircrew categories during the war in Jeff Jefford's "Observers and Navigators" and that's the picture that emerges in the 10 Squadron (Halifax) ORBs of that time. Possibly of interest - when the Squadron switched to Dakotas after VE Day and went out to India, the crews included the Air Bombers as 2nd Pilots for a couple of months until a group of pilots, until then on gliders, became available to take over.