According to Wikipedia: "In 1960, the RAF issued Specification 239 for a strategic transport, which resulted in an order being placed by the Air Ministry with Vickers in September 1961 for five VC10s. The order was increased by an additional six in August 1962, with a further three aircraft cancelled by BOAC added in July 1964. The military version (Type 1106) was a combination of the Standard combi airframe with the more powerful engines and fin fuel tank of the Super VC10. It also had a detachable in-flight refuelling nose probe and an auxiliary power unit in the tailcone. Another difference from the civil specification was that all the passenger seats faced backwards for safety reasons."