From the Latin dorsum, meaning back.

Dorsal can also be an ornamental cloth behind and above an altar.

7 minutes ago, JamesSavik said:

On the classic B-17g bomber, the dorsal turret is often called the top turret. Crewmen much preferred this position to the ball turret on the bottom of the bomber.

 

For good reasons too. If the gears got jammed, you were trapped on the belly of the plane. Not a place to be if the landing gear was kaput. Besides, the top one is easier to climb out of.