Developed during the late 1940s, the Balliol was designed to fulfil Air Ministry Specification T.7/45, replacing the wartime North American Harvard trainer.

[2] A further stipulation by the ministry was the fitting of a three-seat cockpit in a configuration roughly akin to the contemporary Percival Prentice basic trainer.

[2] Within a month of the specification's issuing, Boulton Paul Aircraft had opted to produce multiple proposals in response, as large orders had been anticipated.

Boulton Paul's P.108 proposal, which would become the Balliol, was that of a conventional low-wing monoplane, featuring retractable main undercarriage and a fixed tailwheel.

[1][3] Towards the end of August 1945, Boulton Paul received an order from the Air Ministry, calling for the production of a batch of four prototypes, which were to be powered by the Rolls-Royce Dart turboprop engine.

[5] As the Mamba was not flight-ready at the time of the first prototype's completion, it was instead powered by a 820 hp (611 kW) Bristol Mercury 30 radial engine during its initial test flights.

[1] Early handling trials at MOD Boscombe Down revealed the aircraft to possess relatively gentle and easy flight characteristics across all typical circumstances, with only minor suggestions for improvements being produced, such as the lack of a port-side walkway and directional snaking unbecoming to a gun platform.

Following an extensive evaluation, the Balliol was chosen as the victor over the rival Athena, leading to sizable orders being promptly placed to replace some of the Harvards in RAF service.

A specialised model for the Royal Navy's Fleet Air Arm (FAA), designated Sea Balliol T.21, was also developed.

This was largely due to attitudes amongst staff at the Air Ministry having shifted once again on the topic of training requirements, with the RAF now wanting a jet-powered advanced trainer aircraft instead.

[13] Having anticipated student pilots being particularly rough on the aircraft, it was designed to be both durable and as easy to maintain as possible, featuring around 40 inspection hatches, typically secured by quick-release fasteners, across the fuselage to facilitate this.

For greater simplicity of maintenance, wherever feasible, components were designed to be interchangeable, such as the fin, tailplane, wing tanks, main gear oleos, and other elements.

[15] The majority of exterior covering was light alloy stressed-skin, supported by a combination of subframes and longerons, while the rear section incorporated a monocoque approach.

[16] Most of the wing's structure comprised pressed light alloys, while an auxiliary spar was used to support the trailing edge skinning and flaps and the forward section accommodated the steel box wells for the retractable main undercarriage.