volunteering for missions?

I am writing a memoir of my late  brother's service as a belly gunner in a B-17 during World War II. He was with the 346th Squadron of the 99th BG. He served from about June of 1944 to November of 1944 at which time he had completed his 50 missions. I know that one of the officers on his regular crew did not finish his 50 missions until about 3 months later. Is is likely, or possible, that my brother volunteered for extra missions, or was he probably assigned those misssions. I have his mission list and he flew about one third of his missions with other crews. I would like to hear comments on this topic.
Ralph Ekwall
  ralphomaha@yahoo.com

 
Ralph,
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.

Ralph....My experience in the 99th 348th was toward the end....I flew only with my crew,  (as Flight Engineer), however, the other gunners were assigned to fill in on other crews, and as such, have several more missions than I did. While there were some individuals who volunteered, most were assigned.
 
Are you planning on attending the 99th BGHS reunion in October this year ?
 
Jim :-)
 

Ralph,
Other then crews that came overseas together, I doubt if  most crews finished their tours together. After my first mission I was detached to fly a General for several days, When I returned my crew had flown 6 missions. About halfway through my tour my ball gunner who was radio qualified was assigned to a different crew. I got a new gunner who flew with me until I finished. When the rest of my crew finished I still had 6 missions to go. I then flew as pilot with several new crews that joined the sqdn.