Paradoxically, the Mk.V, which ended up being the most numerous variant of the famous fighter built, was not even an intended development of the design. Indeed, it was only considered because of the abandonment of the Spitfire Mk.III. As the Luftwaffe was continually improving its formidable Messerschmitt Bf 109, the latest version of which, the Bf 109F, clearly outclassed the Spit?re Mk.II, the British had no other option but to rapidly ?nd a successor to the latter. At this time, at the end of 1940, the British did not know what the Germans' were planning and expected them to carry on undertaking daylight raids upon the return of nicer weather. The RAF, therefore, wanted to be ready to counter the new German fighter developments. A solution was soon found by mounting a Merlin 45 (former Merlin III), a simpli?ed version of the Merlin XX, on a slightly strengthened Spit?re Mk.I or Mk.II airframe. Several Spitfires were thus modi?ed, with either a Merlin 45 or Merlin 46, during the first weeks of 1941. The resulting feedback was good and the Air Ministry requested that Supermarine modify, as early as possible, Spit?res already on the assembly lines so they could be put into service as rapidly as possible. This is how the Spitfire Mk.V came to be. In 1941, the Spitfire Mk V progressively became the backbone of the Fighter Command while new squadrons were formed or coverted that year. Two pre-war squadrons, initially formed as Special Reserve squadrons, Nos. 501 and 504, converted to the type in 1941 and the book, made of 36 pages with about 25 photographs and five colour profiles, narrates their operational usage of the Spitfire Mk V.