The Balliol was developed to meet Air Ministry Specification T.7/45 for a three-seat advanced trainer powered by a turboprop engine, competing against the Avro Athena. It was a conventional low-wing monoplane with a retractable main undercarriage and a fixed tailwheel. Pilot and instructor sat side by side ahead of a second student who would watch the instruction given to the pilot. In August 1948, Boulton Paul received an order for four prototypes, to be powered by the Rolls-Royce Dart turboprop, and in August 1946, this was followed by an order for 20 pre-production aircraft, with ten each to be powered by the Dart and the Armstrong Siddeley Mamba turboprop, with delays to development of the Dart meaning that the prototypes would now be fitted by the Mamba. The Mamba was not flight ready when the first prototype was completed, so it was fitted with a Bristol Mercury 30 radial engine for initial testing. Thus powered, it first flew on 30 May 1947. The second prototype, powered by the intended Mamba, first flew on 17 May 1948, the world's first single-engined turboprop aircraft to fly. The Air Ministry had meanwhile had second thoughts about its training requirements, and in 1947 issued a new specification, T.14/47, requiring a two-seat trainer, powered by a Rolls-Royce Merlin piston engine, available in large numbers from surplus stocks, rather than the expensive turboprop engines.