The carrier announced Thursday it’s buying 15 planes from Boom Supersonic with the option to purchase 35 more at some point. Boom’s first commercial supersonic jet, the Overture, has not been built or certified yet. It is targeting the start of passenger service in 2029 with a plane that could fly at Mach 1.7 and cut some flight times in half. That means a flight from New York to London that typically lasts seven hours would only take 3½ hours. Earlier this year, United took a stake in eVTOL start-up Archer Aviation while partnering with Mesa Airlines to order 200...