Avro Lancaster Mk.II Prototype

The Avro Lancaster Mk.II was a variant of the Lancaster powered by four Bristol Hercules (VI or XVI) engines rated at 1615hp at take-off or 1750hp at 15000ft (4572m) high. They were designed by Avro, who produced one of them, the prototype, but the production serial was overtaken by Armstrong Whitworth at Coventry.
One difference between the VI or the XVI was that the VI had manual mixture control that required an extra level on the throttle pedestal and some of them were fitted with a ventral turret just below the spinal one. Some of them were fitted also with an enlarged bomb bay that could carry a payload of 8000lb (3629kg) in bombs. As the ventral turret was barely used at first, many of them were removed to save the extra weight, however, when, in late 1943, the Luftwaffe started to use the Schräge Musik gun system to attack the bombers, some unofficial modifications were made like the inclusion of downward observation blisters or 0.5in (12,7mm) machine guns or even 20mm cannons that fired through the ventral holes of the removed turret. However, those guns, interfered with the bombs' aim when the aircraft had bulged doors.
The prototype made the maiden flight at Manchester on 26th November 1943 and its performance convinced Avro to manufacture the variant, however, the production was delegated to Armstrong Whitworth. In November 1943, after 300 of them manufactured, production of the Mk.II version was discontinued and Armstrong Whitworth switched their production of Bristol Hercules engines for the Handley-Page Halifax Mk.III.

Sources:
1. https://en.wikipedia.org/wiki/Avro_Lancaster
2. Signal Squadron - Aircraft In action 52 - Lancaster in Action