The growing capability of high-performance monoplanes entering RAF service from the late 1930s highlighted the need for an advanced trainer with similar performance, and Miles designed a low-wing monoplane trainer to be powered by the 745 hp Rolls-Royce Kestrel XVI used in the Hawker Fury and Hart biplanes. When the design was submitted to the Air Ministry it was considered premature, but the company continued with construction of the prototype as a private venture, and this, named Kestrel, was flown for the first time on 3rd June 1937. It was very soon demonstrating a maximum speed only about 15 mph below that of the Hurricane, and had handling characteristics similar to those of the Hurricane and Spitfire. With no alternative in prospect, the Air Ministry ordered the Miles trainer on 11th June 1939 under the designation Miles M.9 Master, but requested changes, including use of the de-rated 715 hp Kestrel XXX, which reduced the maximum speed to 70 mph below that of the Kestrel prototype. Even then the Master still had by far the highest performance of any trainer in the world, with a top speed of 226 mph. at 14,000 feet. Pilots trained on the Master had no difficulty in graduating to the Hurricane and Spitfire fighters, and the majority of the men who won the Battle of Britain - probably the most decisive battle in recorded history - were initiated to high-speed flying on the Master.
The actual production of the Master was expedited by the use of a track assembly system, the first of its kind in the world, and machines came off the line in ever-increasing numbers up to and into the war years, with the first of 900 examples of the M.9A Master Mk I being flown on 31st March 1939. Eight months later Miles flew the first M.19 Master Mk II, differing by having a 870 hp Bristol Mercury XX radial engine, which had been substituted at Air Ministry request because the supply of Kestrel engines was dwindling. The Ministry then discovered it had no stocks of Mercury engines and the 825 hp Pratt & Whitney Twin Wasp Junior was installed in a modified airframe to produce the M.27 Master Mk III. However, both Mercury and Twin Wasp Junior engines were used in production aircraft, the eventual number built totaling 1,747 Master Mk IIs and 602 Master Mk IIIs. To these figures can be added 26 M.24 Master Fighter air craft, each armed with six 0.303-in machine guns. These last Masters were hurriedly converted to single-seat fighters during the darkest days of the Battle of Britain, when there were fears that the supply of fighter aircraft might be exhausted before victory was assured. In addition to use by the RAF a number of Master Mk IIs were supplied to Egypt (26), Portugal (1), South Africa (450) and Turkey (18). One was transferred to the US Army Air Force and one Master Mk III went to the Irish Air Corps. When production ended a total of 3,227 had been built, the Miles Master proving to be the most significant trainer of indigenous design to serve with the RAF during World War 2.