Boom Supersonic can now add United Airlines to its list of supporters. The carrier plans to purchase 15 of Boom’s Overture planes once the aircraft meets its “demanding safety, operating and requirements.” The agreement includes an option for United to buy an additional 35 craft for a total order of 50 jets. While Boom recently unveiled its XB-1 test plane and plans to fly it for the first time later this year, the company has yet to build a working passenger jet.

If everything goes according to plan, United will start operating Overture passenger flights sometime in 2029. Among other potential routes, it envisions the Mach 1.7 plane flying from Newark to London in under four hours and San Francisco to Tokyo in approximately six hours. On a modern jet plane, those flights typically take about seven and 11 hours. The two companies claim Overture will be a “net-zero carbon” craft from day one on account of the fact it will use sustainable aviation fuel. They say they plan to work together to increase production and supply of the fuel but didn’t detail how they’ll go about that goal.

Even with United and other carriers like Japan Airlines helping Boom get its first passenger jet off the ground, the startup has a tough challenge ahead of it. Many of the same obstacles that grounded the Concord in 2003 still exist today. For one, the Federal Aviation Administration has a longstanding ban preventing civilian aircraft from flying faster than the speed of sound, limiting the routes craft like the Overture can fly. Planes that fly faster also use more fuel. Not only have airlines historically favored aircraft with better fuel efficiency, but there are also even examples of some carriers ordering their pilots to fly slower to save on costs.