The Boulton Paul P.131 was a design for a jet trainer, designed in response to Specification O.R.AIR/37, but that didn’t enter production.
In 1951 Boulton Paul had produced the private venture P.119 jet trainer, which reached the mock-up stage. This wasn't taken on by the Air Ministry, but it did give the company experience that they were able to use on the P.131.
The P.131 was powered by a Bristol Orpheus engine. The air intakes were in the wing roots (on the P.119 they had been in the side of the fuselage). It was based on the earlier P.124, but was modified to fit an Australian specification.
Like the P.119, the P.131 had been designed to fit between the Balliol and operational jet aircraft. The Air Ministry then decided to go for an all-jet training system, and went for the Jet Provost and the Vampire trainer, leaving no place for the P.131. The RAAF also went for the Vampire.