The Short 31/26 was an unrealised 1927 project to develop an autogyro flying boat to meet UK Air Ministry specification 31/26. It is also known by its Cierva design number Cierva C.13 or Cierva C.14. 
Design.
On 11 July 1927, the UK Air Ministry issued specification 31/26: "Gyroplane boat seaplane (Jupiter engine)". The specification called for "the design and construction of a three-seater gyroplane boat seaplane, embodying the principles of the La Cierva autogiro."
The specification goes on to describe a flying boat with positions for a pilot, observer, and gunner. The gunner was to sit in a forward cockpit and be armed with a Lewis gun mounted on a Scarff ring. The aircraft was also to be able to carry twelve bombs that could be launched via an RL tube. A Bristol Jupiter VIII radial engine was specified for the powerplant. The aircraft was to be capable of and have a stall speed not greater than . In addition to its rotor, it was to have fixed wings.
Development.
Development was conducted in partnership between Juan de la Cierva and Short Brothers chief designer Arthur Gouge. A two-bladed main rotor was chosen, to be fitted on a pylon above a metal hull. Sponsons were to be fitted for stability on the water.
A model was constructed and tested in the Short Brothers tank at their Rochester factory, but development progressed no further. The experiments showed problems with vibration, which aviation historian Peter W. Brooks notes "would almost certainly have proved unsuperable at this period."