United Airlines sees a supersonic future

United Airlines aims to bring back supersonic travel before the decade is over with a plane that has yet to be built.

The airline said Thursday that it plans to buy 15 jets from Boom Supersonic with an option for 35 more once the start-up company designs a plane that flies faster than the speed of sound while meeting safety and environmental standards.

United hopes to carry passengers on the plane in 2029. The airline said the plane will reduce flights between London and the New York area to just three and a half hours and make Tokyo only six hours from San Francisco.

United declined to discuss financial terms of the deal. An executive said the airline put down a deposit, which he termed an unusual show of faith in a plane that won’t fly for several years, but would not discuss further details.

It has been nearly two decades since the last flight of the supersonic Concorde, which British Airways and Air France began using in 1976 to zip passengers in luxury across the Atlantic. The last one was retired in 2003, three years after an Air France Concorde crashed into a hotel shortly after takeoff from Paris, killing everyone on board and four people on the ground.

Several companies are working to come up with new supersonic jets that would be more economical on fuel — and create fewer climate-changing emissions — than the Concorde.

Boom is working to develop an 88-seat plane it calls Overture, which it says will be the first supersonic airliner to fly on so-called sustainable fuel. A spokeswoman said that a prototype will make its first test flight later this year or early in 2022.

The Denver company said the plane will be capable of speeds up to 1.7 times the speed of sound, or about 1,300 mph. That is slower than the Concorde but much faster than current airliners,...