The Merlin was designed by F.G. Miles of Philips and Powis, as a result of collaboration with G. Birkett of Birkett Air Service Ltd, and Tata Sons Ltd of India. It was based on the three-seat and four-seat versions of the M.3 Falcon, but with wider fuselage and consequent increased wing span. It was a low-wing monoplane, with fixed main undercarriage in trouser fairings plus a fixed tailwheel. Construction was mainly of wood, with spruce frames and three-ply birch covering, and the wings had hydraulically operated split flaps. It was powered by a 200hp de Havilland Gipsy Six piston engine. It was first designated M.4 Merlin, but all four examples were completed as M.4A Merlin, with fixed pitch propeller instead of the intended variable pitch unit.[1][2][3]