The Boulton Paul P.109 was a design for an advanced trainer powered by a Bristol Perseus engine.
The P.109 was designed in response to specification T.7/45, for a complimentary advanced trainer to replace the wartime Harvard and Master trainers. This was to be powered by a turbo-prop engine, but also had to be capable of using an improved Bristol Perseus engine. The specification was issued on 16 March 1945. Boulton Paul submitted their turbo-prop powered design in mid April, and the Perseus powered version two weeks later.
Both versions were low winged monoplanes, with all metal construction. The pilot and first student sat side by side, with the second student sitting sideways behind them. The turbo-prop version was the Boulton Paul P.108, which entered production as the Balliol, and the Perseus powered version was the P.109.
In August 1945 T.7/45 was reissued, without the requirement for the Perseus engine. As a result work on the P.109 came to an end, although the earlier version of the P.109 and the eventual production version both used piston engines.