United Airlines wants to bring back supersonic air travel. The company said on Thursday it would buy ultra-fast jets from Denver-based aerospace company Boom Supersonic. Under the agreement, the airline will purchase 15 of Boom’s aircraft once they meet United’s safety, operating and sustainability requirements. Supersonics have come under criticism from environmentalists for burning more fuel per passenger than regular airliners. But United said the aircraft would be optimized to use 100% sustainable aviation fuel from its entry to service, which is not expected until 2029. Boom’s so called ‘Overture,’ a supersonic aircraft would cut transatlantic flying time by 50% to about three-and-half hours. If it takes the skies, the jets would be the first supersonic aircraft to fly since 2003 when Air France and British Airways retired their Concorde service after 27 years.