Memphis Belle was NOT the first 8th AF B-17 to complete 25 missions. That honor belongs to Hell's Angels. It seems that the 8th's Public Relations Office screwed up. Hell's Angels completed 25 missions on May 13th, 1943, which was six days before the Belle did. Capt. Morgan only flew 17 or 18 of the 25 anyway, missing some due to sickness or flying a different B-17. Capt. Baldwin flew 27 of the 28 missions flown by Hell's Angels.
You can check out the mission schedules for both
here. So, the actual answer to your question is Memphis Belle.

My regards,
Widewing