In 1938 the Air Ministry informed the Miles organisation that stocks of Kestrel engines for the Master were running low, but that Bristol Mercury motors were available in quantity. The Master was then modified to take this radial, and as Master II successfully passed its trials in November, 1939.
With everything set for quantity production it was suddenly discovered that the Mercury stocks were not as large as was at first thought, and further modifications were put in hand, this time to accommodate an American radial engine, the 825 hp Pratt and Whitney fourteen-cylinder Twin Wasp Junior, the new version being known as Master III. This was no sooner accomplished, however, when lots of Mercury engines
materialised, and Master II went into production after all.
In 1940 Master III also went into production, at a Miles shadow factory, and in 1942 numbers of the Master II were converted for glider tug duties. This latter modification involved the installation of a hook and release mechanism in the rear fuselage, and removal of a portion of the bottom of the rudder to give cable clearance. Altogether nearly a dozen variations of the Master were constructed.