In 2013, a feature article appeared on the website of legendary aerospace and defense giant Lockheed Martin that quickly had aviation buffs talking. The piece, titled “Speed is the New Stealth,” claimed that a new hypersonic aircraft was under development at the company’s famous Skunk Works facility, which would soon set a new standard for speed and stealth in American aerospace technologies. “That’s because today,” the article explained, “engineers are developing a hypersonic aircraft that will go twice the speed of the SR-71.” “It’s called the SR-72.” It was a remarkable revelation since 2013 marked nearly 23 years since the...