United Airlines hopes to bring back supersonic travel before the end of this decade. United said Thursday that it reached a deal with startup aircraft maker Boom Supersonic to buy 15 of Boom’s Overture jets. The planes haven’t been built yet, but Boom says they will fly at 1.7 times the speed of sound, or around 1,300 mph. That would be fast enough to zip passengers from London to New York in three and a half hours. It’s been nearly 20 years since the last Concorde supersonic plane was retired.