A routine training flight by the U.S. Navy’s E-6B Mercury “doomsday plane” captured the attention of sky-watchers this week in Colorado. The four-engine command-and-control plane based on Boeing Co.’s 707 airliner on Wednesday took off from Travis Air Force Base in California and circled over Denver before continuing on to Tinker Air Force Base in Oklahoma. It was apparently the hour-long racetrack holding pattern that turned the heads. “Did you see this today?” a newscaster at the KMGH, the local ABC affiliate, asked during a broadcast. “There was this plane just circling the metro, circling and circling, and many of...