Lancaster: Armament Summary
Early Lancaster's also were also equipped with a Ventral (mid-lower) Turret. Although some production Lancaster's reached the squadrons with this turret installed. It was soon decided, by those not flying the aircraft in hostile skies, that the turret was almost useless; and it was were soon removed from both the operational aircraft and those still on the production lines.
The deletion of the ventral turret however, left the aircraft underside undefended and blind. More than one Lancaster would fall victim to a night-fighter approaching, undetected, from underneath.
It should be noted that the approximate location of the ventral turret was eventually used for the H2S radar assembly. Some squadron's took it upon themselves, to install a single 0.50 calibre machine gun in the position.
Late in the war, the Lancaster's lacking in defencive armament finally began to be corrected. Aircraft began to arrive with two 0.50 Cal. machine guns located in both the Rear and Mid-Upper turret positions. Although, the 0.50 calibre had a far greater range and more destructive hitting force than that of the 0.303, it still did not equal the German 20mm (0.75") cannons which were installed in most night-fighter aircraft.