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