United plans to buy 15 Supersonic planes Premium MATT GROSSMAN , The Wall Street Journal The planes haven’t been built yet, but the deal with Boom Supersonic carries the promise of a return for faster-than-sound air travel Share Via Read Full Story United Airlines Holdings Inc. took a step toward returning supersonic flying to passenger travel, unveiling an agreement to buy faster-than-sound airliners from Boom Technology Inc., an upstart aerospace company that says its planned aircraft could carry travelers by the end of the decade. United’s agreement to buy 15 of Boom’s Overture aircraft is conditional on whether the design—which hasn’t yet been built—can meet safety, operational and sustainability standards, the companies said Thursday. Even so, the announcement suggests that one of America’s largest airlines sees the potential for a return of supersonic air travel, which disappeared with the retirement of the Concorde almost 20 years ago.