The B-25 Mitchell doesn't have a belly gunner, which would seem quite dangerous. Why didn't it have one?
The production B-25B, B-25C, B-25D, and some B-25G models did have retractable remote control belly turrets. They were often removed in the field because they were ineffective and disliked by the crews.
The lower turret was officially deleted in the middle of the B-25G production run and continued with the B-25H and B-25J production.
More info on the B-25 here: B-25 History.org
My research has shown that the turret was operated through a panaflex prism periscope that caused such intense vertigo and nausea in its' user that is was rarely used and often removed. In the Pacific Southwest, the turrets were immediately removed and replaced with fuel tanks to increase range and also because monsoon rains turned airfields into mud which covered the gunsight on takeoff rendering the turret useless.
My guess is that that conventional fighter aircraft couldn't attack from below (because they have forward-firing guns and can't fly vertically upwards) -- so belly-mounted guns are the only guns you don't need.
The Germans introduced upward-firing guns (Schräge Musik) so that they could attack from below (to take advantage of this undefended approach), but these didn't come into service until late in 1943, and (being on nightfighters) weren't discovered by the Allies for the next year or more.