The Boulton Paul P.85 was a design for a naval version of the P.82 Defiant turret fighter, but was rejected in favour of the Blackburn Roc.
In 1935 the Fleet Air Arm issued Specification O.30/35, which called for a turret fighter capable of being operated off an aircraft carrier. Unsurprisingly Boulton Paul offered a version of their Defiant, using a Bristol Hercules HE-ISM engine to satisfy the naval preference for radial engines.
The Navy chose a turret version of the Blackburn Roc, the Blackburn Skua. This wasn't entirely bad news for Boulton Paul, who got the contract to fit their A type turret in the Skua, and then built the modified aircraft. The company was so involved in the Skua that they gave it an internal Boulton Paul project number, P.93.
The Skua was something of a disaster in naval service, forcing the Navy to quickly develop interim fighters to replace it. Although the Defiant wasn't a success on land, a naval version might have been more useful. Single seat versions of the Defiant were considered after the turret fighter concept failed in 1940, and a similar conversion of a proper naval Defiant might have given the Fleet Air Arm a good carrier fighter before the more able American designs arrived later in the war.