The Boulton Paul P.91 was a design for a heavy bomber that was produced to the specification that eventually produced the Handley Page Halifax and Avro Manchester and Lancaster.
The P.91 would have been a twin engined bomber, powered by two Rolls Royce Vulture engines (and thus similar to the Manchester). It was designed in response to Specification P.13/36. Like the similar Boulton Paul P.90, it would have had an unusual tail turret, with four guns mounted in two pairs, each on an articulated winglet low mounted on the sides of the fuselage, just behind the tail. This design gave the rear gunner a very wide range of fire behind and below the aircraft. The P.91 would also have used turrets armed with multiple cannons. It would probably have used the same cylindrical cheek mounted bomb bay as the P.90, which was designed to reduce the size of the fuselage.
The P.91 didn't reach the prototype stage. The contest was won by the Avro Manchester, which later evolved into the Lancaster after the Vulture engines failed, and the Handley Page Halifax.