A quick Google turns up:
B-29 -- 2513 produced; 1620 in Wichita, 536 in Omaha, and 357 in Marietta. (Doesn't include the B-29As built in Renton).
Since production didn't start until September 1943 in Wichita, and 1944 in the other two locations, you are looking at mass production of over 100 per month.
Thanks. That would seem to satisfy my definition of mass production.