5. Hiller X-18

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Conceived of in 1955 by Hiller Aircraft Corporation, this odd-looking aircraft could easily pass for the predecessor to the modern V-22 Osprey that is currently in service. The first of 20 test flights was conducted at Edwards Air Force Base in California before the program was cancelled in 1964 and the only model of its kind ever built was disassembled and used for scrap.