RE: volunteering for missions?
I can only speak for my Group (381st BG, 8th AF), but we did not have to volunteer to fly as a fill in with another crew. If man was unable to fly, a spare gunner would be put in his place by assignment. We had a pool of spare gunners that would normally supply a needed gunner. In some cases, such as the Ball Turret (belly gunner), where operating knowledge and experience was important, an experienced gunner would be assigned from a crew that was not flying that day. I flew 2 missions as a spare for other crews and finished ahead of the rest of the crew except the Navigator who had flown as spare 4 times.
S/Sgt, 35 missions
Ball Turret Gunner, Goldin crew
381st Bomb Gp., 533rd Bomb Sq.
US 8th Air Force
Author: "The View From The Bottom Up" memoir