6. Douglas X-3 Stiletto

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This super-sleek-looking jet may have been designed in the 1950’s but it still looks pretty cool some 60 years later. It’s primary purpose was to investigate the requirements of supersonic flight. It wasn’t a particularly successful design and never managed to deliver the data sought by researchers on the properties of Mach 2 speeds. However, it still provided useful data that was helpful in understanding a flight hazard called roll inertia coupling.