I am just assuming that the Lancaster "lacked" a ball turret to accommodate a larger bomb-bay.? Perhaps, also, because they flew mostly at night; the thought was a bottom gun was not needed.? At any rate, after it was all said and done, did the absence of a bottom turret prove to be much of a problem for the Avro Lancaster.? Thank You
This isn't my area and without doubt someone with vastly more knowledge on the subject will pop along soon to answer your question but the lack of an underside turret led the Germans to exploiting this blind spot with the use of upward firing cannons, the so called Schräge Musik. And I seem to recall that some Lancasters were fitted with a single underside gun facing the rear but whether this was just a trial or not I don't know.
Just googling gives the following Lancaster FN-64 ventral/belly turret (For Mk1 & Mk2) It gives fairly good answer. One point not necessarily mentioned in the thread, I read only a part in the beginning, is that night sky is "always" lighter than "blacked out" wartime land, so it is much easier to see a plane against night sky than against land, so the attacking night-fighter had a clear edge against a gunner scanning through a periscope with its small field of view and a periscope always absorb some light.
Lancaster Restoration And Fabrication - FN64 Turret Able Engineering Midlands ltd. Welding, fabrication and machining. Parts supply for the 101 Land Rover - FN64 Ventral Turret Restoration group looking to restore the last known existing Lancaster ventral turret
I believe that when previously looking at this book online, The Avro Manchester - The legend behind the Lancaster by Robert Kirby, it showed experiments to fit ventral turrets to the Avro Manchester, with photos.