That's a little tougher than the B-17, Glen!
I'd guess that the first 25 mission B-24 would have been a part of the Halpro Project, they were the first U.S. Libs in long term combat. There were a few LB-30s out in the Pacific at the start, and a few in the Aleutian area also. Of course, the RAF must have had a bunch of early 25 mission Liberators.
The B-29 question would depend on how missions were counted in the CBI and will take a little research. I'll dig into the 58th Wing History a little to see if I can find something. For some reason, it seems that a 40th Bomb Group airplane took the honors, but I'm probably incorrect.
Scott