Armed with six 0.303 inch (7.7 mm) machine guns, it was intended for mass production as an emergency fighter during the Battle of Britain; this model did not ultimately see combat.

Ordinary trainer models could also be fitted with armaments, including a single .303 in Vickers machine gun and eight bombs, for training purposes only.

The M.9A Master I was based on the privately developed M.9 Kestrel trainer that was first demonstrated at the Hendon Air show in July 1937, although this aircraft never entered production.

Alterations included the installation of a lower-powered (715 hp (535 kW)) Kestrel XXX engine, of which there were large surplus stocks available, along with extensive revisions to the airframe, which involved the adoption of a new cockpit canopy, a modified rear fuselage and tail, along with the repositioning of the radiator from underneath the nose to the underside of the wing's centre-section.

[3] Miles had designed the Master to fulfil their vision of an effective trainer aircraft, being one that could match the performance of, and possess similar characteristics to, the frontline RAF monoplane fighters of the day, these being the Supermarine Spitfire and the Hawker Hurricane.

[8] In a typical trainer configuration, the Master was equipped to carry eight practice bombs, plus a single .303 in Vickers machine gun that was mounted in the front fuselage.

[citation needed] A total of 3,249 Masters were built by Phillips and Powis Aircraft Limited (as Miles was known until 1943) at Woodley, Berkshire; South Marston, near Swindon, Wiltshire; and Doncaster.

[1] The mass production of this aeroplane at Woodley required a major expansion of the original Phillips & Powis factory, which was officially opened on 27 January 1939 by the Secretary of State for Air, Sir Kingsley Wood.

[4] Initial models used the Kestrel XXX engine; capable of providing up to 745 hp (555 kW), this powerplant enabled the aircraft to achieve a maximum speed of 296 mph (477 km/h),[2] which reportedly made the Master as fast as the single-seat biplane fighters of 1935.

[11] While the Master had incorporated relatively advanced aerodynamic characteristics (intended to mimic frontline fighters) for a contemporary trainer aircraft, it used a conventional structure, comprising an oval-section fuselage covered by a plywood skin, featuring a semi-monocoque approach.

[3] Forward of the tandem cockpits, the nose is strengthened by a metal former that provides protection against nose-overs, a common occurrence amongst trainee pilots when flying aircraft with 'tail dragger' conventional undercarriage.

[11] The Kestrel engine of early-built aircraft incorporated various auxiliary drives to power both vacuum and hydraulic pumps, along with an air compressor and a 500-watt electrical generator.

The forward windscreen is composed of moulded Perspex and furnished with a reflector-type gun sight, providing an optically-perfect view of a target.

[13] Two small panels can be opened to aid visibility while flying in poor weather conditions, sun blinds are also incorporated.

[citation needed] RAF stocks were frequently diverted to support several of the air services of the Allies and other non-hostile nations.

[citation needed] Despite having been produced in the thousands, there are no known surviving aircraft of the type today, although a few outer wings and other parts are held by several aviation museums in Britain.