Deciding who got to be a pilot or co-pilot

Hello,
This is a question that has been on my mind for some time.
At what stage in training was it decided which bomber pilots would become First Pilots and which would become Co-pilots? And what factored into the decision? Was a co-pilot disappointed with being given that designation?
I do realize that in the course of combat a co-pilot would often end his tour as a First Pilot but it might take him many missions to get to that point.
Elsewhere in this forum, B-17 pilot Jules Horowitz has said that he felt a co-pilot had the most frightening job on the plane in terms of feeling helpless. 
Were there any pilots who preferred to become co-pilots?
I'd love to hear from actual WWII vets on this subject, and Jules Horowitz in particular, if he wouldn't mind sharing his knowledge with us.
Thanks.