The plane's engineer manned the top turret. When not occupied as a gunner the engineer tended to the aircraft's many electrical and hydraulic systems and moved fuel between tanks as needed to keep the plane in balance.

The Martin top turret was used on the AAF's B-24 Liberator heavy bomber and the B-25 Mitchell attack bomber as well as the Navy's PB4Y Liberator and Privateer bombers.

Dad on top of the crew's B24L,
leaning against the Martin top turret.
Turrets are most often associated with bombers and flying boats, but the Navy's Avenger featured a turret. The avenger was a dive bomber and the gunner provided defensive fire with a single .50 caliber Browning machine gun.
The Martin 250H was used as both a top and rear turret in various flying boats.

Martin Mariner.
The Bendix 250CE was used on the Navy PBY Mitchell in place of the Martin turret used by the Air Force.