Interesting note on the Spitfire... The British were working on long-ranged armed versions of the Spitfire before the Mustang arrived. However, this work was halted when the Mustang arrived and so beautifully filled the role of long-range escort. The British test pilot Jeffrey Quill flew a modified long-range Spitfire Mk IX, and I quote from his book:
"Long range escort was the role in which the Merlin Mustang was particularly excellent because of the large load of fuel it was able to carry. True, the Spitfire Mk VIII, in service in 1943, was carrying additional fuel in its wing roots and also in jettosonabale tanks under the fuselage, but it was serving overseas and the problem of accommodating larger loads of fuel in the Spitfire at home was acute. The only available space was in the fuselage behind the pilot, but a tank of significant size there would have a major effect on the centre of gravity.
However, it seemed to both Joe Smith and myself that, for the purpose of escorting bomber formations in daylight, a degree of longitudinal instability in the early stages of a sortie would be acceptable. Therefore the fuel in the rear fuel tank could be used for take off and climb and during the early stages of the sortie, the main tanks and wing tanks remaining full. In this case the centre of gravity would be moving forward to an acceptable position by the time the aircraft reached hostile airspace. It was decided therefore to embody a rear fuselage tank in a derivative of the MkXIV shortly due to come into production, the Mk XVIII.
In the meantime a 75-gallon tank was fitted in the fuselage of a Mk IX behind the pilot and we also fitted a bob-weight in the elevator circuit, so what with this and the large horn-balance on the elevator we hoped for the best. However the best and most expedient way to test this aeroplane was to fly it a good long way and see how everything worked. So I took off from High Post on Salisbury Plain with all tanks full, carrying a 45-gallon drop tank in addition, and set off at economical cruising boost and RPM in the general direction of Scotland. The weather was unsettled, so I decided to fly at low altitude which was not, of course, a favourable height for optimum air miles per gallon: but I thought that if I could fly a distance equivalent to John o'Groats and back non-stop at that rather unfavourable height, keeping to the east of the Pennines and the Grampians, it would be a useful demonstration.
The aeroplane was unstable to start with, but as soon as I had used up the rear fuselage fuel the handling was back to normal and I settled down to a long and enjoyable flight over a great variety of countryside from Salisbury Plan to the Moray Firth and back again, all below 1,000ft. In distance, and not taking into account the various diversions for weather and terrain, it was the equivalent to flying from East Anglia to Berlin and back. It took five hours.
This flight demonstrated, if nothing else, that there was no fundamental reason why the Spitfire should not be turned into a long-range escort fighter provided that certain problems could be solved.
A demonstration of this basic fact was also given by the Americans. They had two Mk IX Spitfires at Wright Field and by local modification they added two Mustang overload fuel tanks under the wings and some additional fuel inside the wings. They flew them across the Atlantic by the Northern Route via Greenland and Iceland and eventually they were thoroughly examined by the Supermarine design department. Unfortunately some of the structural modifications carried out were detrimental to the strength of the aircraft and so could not be considered for production."
