Might be a B24. In some cases, in particular when operating from Pacific airfields where it might get muddy. they did away with the ball turret and had a "tunnel gun" in that position in a similar fashion.
The more 'slab sided' look to the fuselage makes me think so. Too big inside for an A20 and the early 17s had a "tub' under the fuselage for that gun.
Dan/Slack