(Reuters) - Boom Supersonic on Wednesday unveiled its first demonstrator aircraft X-B1, which is scheduled to begin flight testing next year, in a milestone for the U.S. startup planning a commercial airliner that can conquer the sound barrier. A handful of U.S. companies here are vying to bring back supersonic passenger travel which died out with the Anglo-French Concorde"s retirement in 2003. Today’s planned supersonic jets, while quieter and more fuel efficient than the Concorde, are under pressure from environmentalists and airports to meet noise levels and carbon emissions standards for conventional planes. Denver-based Boom said in a release that XB-1 will undergo a carbon-neutral flight test program which is to start next year in Mojave, California. XB-1 has a 71 foot-long (21.6 m) fuselage, a carbon-composite airframe and three GE-designed J85-15 engines, the statement said. The aircraft’s first flight is targeted for the backhalf of 2021, with entry-into-service of the company’s supersonic airliner Overture expected by the end of the decade, Boom Chief Executive Blake Scholl said in an interview. Overture, a supersonic airliner with 65 to 88 seats priced initially at business class fares, would cut transatlantic flying time in half to about three-and-half hours. The company says it has orders from Japan Airlines Co and Virgin Group. Scholl said he expects the aviation market, hard-hit by the coronavirus pandemic, to rebound by the time Overture comes to market. “The nice thing about supersonic jets is if you’re concerned about time on airplanes, less time is better,” he said. Scholl founded Boom in 2014 after serving in leadership roles at Amazon and Groupon, according to the company’s website.