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（CEB|SHL Talent Measurement） The SR-71 is a semi-retired U.S. Air Force reconnaissance plane. This sub-orbital jet was used by the United States predominantly as a spy plane in the decades before satellites were used to perform the same function. The SR-71 was first flown in 1964 for stealth and to avoid radar detection; however, it was capable of flying so high and so fast that it was generally undetectable from the ground. It also had no capacity for combat because any changes in its speed or shape would undermine its ability to be “invisible." As a result, some described it as a "flying engine." One pilot reported that the plane was fired at over 1,100 times during its active tenure, yet was never hit. Also known in the United States as the "Blackbird" and in Japan the SR-71 was able to fly at speeds up to 2,200 miles per hour, equivalent to more than three times the speed of sound. A camera loaded with high-speed film was mounted in the belly of the and took pictures from high altitudes to provide the U.S. military with information about the activities of its adversaries during the Cold War. The cameras were so sophisticated that they could photograph a golf ball from 80,000 feet. The total number of Blackbirds ranged from 30 to 40 before its eventual retirement. A few were reactivated by NASA in 1996 for research, while some now are used by the U.S. Air Force for display at air shows and in museum such as the Smithsonian Institution. Referring to the above, in what year did the United States begin flying the SR-71?