Avro Lancaster B.II

The B.II was born of a supply problem: Merlin output could not keep up, so the Bristol Hercules radial was adopted instead, in its VI and XVI forms. Armstrong Whitworth built 300 of them. There was one notable practical difference between the two engine versions: the Hercules VI used manual mixture control and needed an extra lever on the throttle pedestal.

Early aircraft carried the FN.64 ventral turret, quickly removed for the same reasons as on the rest of the type: the periscope left the gunner without peripheral vision and traverse was too slow. That empty position proved providential when the Luftwaffe took up upward-firing Schräge Musik attacks. Because the B.II's bulged bomb bay interfered with the H2S radar — which on other Lancasters occupied exactly that ventral station — the opening stayed free and squadrons improvised 20 mm cannon or .50 calibre mountings there, until Modification 925 officially authorised a .303 machine gun in the same place.

Three different bulged bomb bays were used: the prototype's narrow bulge running from just aft of the cockpit to the turret, a full-width bulge over the same length on early production aircraft, and doors bulged along their whole length on late ones.