For an aircraft that manages to last the war and is constantly upgraded and is still used for a long time after i would have to say the Supermarine Spitfire. It is constantly upgraded and fulfills many different roles from fighter to a photo recon bird. It also accomadated many upgrades such as the powerful Rolls Royce Griffon engine and the change to a bubble canopy, thus curing one of its early faults. its only main fault was a lack of range but this was inherent in its design as it was originally designed as a interceptor fighter not a escort fighter like the mustang. It was a product of its era in that respect as air forces believed that the bomber would always get through and did not need fighter protection. The spitfire also accomadated many differect wings to enable it to operate at different altitude. this show a great achieve in the airframe design that it could incorporate these modifications. it was also succesfully used in many different condition arounf the world, from the dust of the desert to the jungles of burma. it was also succesfully modified to operate of aircraft carriers. In my opinion a truely magnificent aircraft.

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Thoughts on Military History
Cornish Scale Modellers
Ross' Military Modelling and Art