The Navy used the B-24 in two versions the PB4Y-1 which was a regular twin tailed B-24 and the -2 version. ABout 850 -2 models were made mostly at the end of WW-II. The single tail version of the B-24 was slightly more efficent than the twin tail version but not enough to go thru the trouble of changing the production lines in the middle of the war.
Almost forgot, the reason for the two top turrets was the Privateers were used in a lot of low level attacks on shipping. If enemy fighters appeared the PB4Y would drop down to the surface of the ocean or land. This tactic put the attacking fighters at a disadvantage and would of course negate the need for ventral protection.
Ah what the heck, I went and found a PB4Y-2 pic from 1949 showing the top two turrets and this one has Emerson nose and tail turrets, I think. A small number of PB4Ys are still in service, I think, as Fire Bombers. Hawkins & Powers in Wyoming operated about half a dozen Privateers as recently as 2002.
two turret guns on top. Wonder why?
Probably because they were flying singly on recon/intel grathering and needed to extra firepower.