The Boulton & Paul P.11/ Type XXI was a design for a two-seat amphibian aircraft, produced in response to RAF Specification XXI.
Specification XXI called for a two-seat amphibian that could operate from land bases, from the sea, or from aircraft carriers.
The main evidence for the design comes from Boulton & Paul drawings labelled Type XXI, and the P.11 designation is tentative, and based on a gap in the recorded P numbers.
The P.11 was a two-bay biplane, powered by one 475hp Napier Lion V engine. The crew of two sat just below the upper wing, which was pierced by a circular cut-out and a rear cut-out. Both were intended to improve visibility, and the circular cut out was also the pilot's entrance. The fuselage was also designed to improve visibility, with a hump in the central section. The pilot had a fixed forward firing Vickers gun, the gunner a Lewis gun on a Scarff ring.
Boulton & Paul built a mock-up of the P.11, but the prototype order went to the Fairey Pintail. The Fairey aircraft didn’t enter British service, but three were ordered by the Japanese Navy.