As far as I recall, I don't believe Jimmy had any one aircraft in particular assigned to him, rather between the various missions he flew and commanded, which were spread out at times, he may have piloted a few different Liberators.
I have read an article that stated that Stewart piloted a B-24 by the name of "Nine Yanks and a Jerk" on his first mission, which was a raid on Kiel. The aircraft was named by a previous crew. I believe the photos on this site are of the same aircraft:
http://www.pbase.com/charliedoggett/family_1940s_earlThe website "B-24 Best Web" has him referenced to have flown the B-24 "Male Call", as seen here:
http://www.b24bestweb.com/malecall-v2-1.htmhttp://www.b24bestweb.com/malecall-v2-3.htm