In most of the flying scenes we see the gun barrels of the .50 caliber Browning machine guns that were mounted in the B-25 nose, but from the interior shots...no machine gun. I am sure they were CGI...all is seen are the barrels. in a real B-25, the bombardier would have manned a real one.
When Yossarian and the others return from leave in Italy, after the C-47 touches down on the runway, a shot of the landing gear is inter-cut; however, the landing gear is clearly that of a B-25 which, although prevalent in the show, are not otherwise included in this particular shot.