Made in the USSR: Reverse Engineering the Boeing B-29 (2001)
airspacemag.com
airspacemag.com
It puzzles me, and English is my first language! What does it mean?
"Tramp" here probably means harlot (consider the cheesecake pinup nose-art common on WWII-era US bombers, as alluded to in the article.)
I'm guessing it's actually akin to "hangar queen," an aircraft that's grounded for maintenance to an unusual extent [1].
EDIT: Or, it might be an analog to a tramp steamer, i.e., a merchant ship that goes from port to port to port carrying whatever cargoes the ship's officers can scare up as they go. [2]