Just found a cutaway of a Do17Z I forgot I had! The bombs were suspended horizontally, on two vertical 'beam' type racks, but the cutaway does not show the bomb door arrangement. Spring-loaded doors were not unusual. For instance, the Blenheim had wooden doors, which were spring-loaded to aid in closing. Once the lock was released, the doors 'snapped' shut.
The He111 doors were cranked or hydraullically opened, the spring-loaded areas being the baffles at the front of each bomb cell, designed to provide 'smooth' air, to prevent the vertically suspended, tail down bombs from fouling the doors, bays or under fuselage when released. Again, Guttorm can probably advise further, as he also has a full size, real, He111 to play with !!!