Startup Boom Supersonic has closed a $100 million Series B investment round to finance continued work on its Mach 2.2 commercial airliner—now called the Overture. The 55-seat aircraft is planned to enter service in the mid-2020s.

The latest round includes $56 million in new financing, plus previously announced strategic investments, and takes the total raised to $141 million. Denver-based Boom has said it expects development of the Overture to cost about $6 billion.

The Series B round was led by educational and environmental fund Emerson Collective and includes existing early-stage investors Y Combinator Continuity, Caffeinated Capital and SV Angel. Boom also has backing from individual investors Sam Altman, Paul Graham, Ron Conway, Michael Marks and Greg McAdoo.

Caffeinated Capital and Y Combinator participated in Boom’s $33 million Series A round in March 2017, which raised funds to build the third-scale XB-1. Construction of the manned demonstrator has been delayed, but assembly is now under way and Boom expects the aircraft to fly this year.

“Selection efforts are ongoing for the manufacturing site for Overture, which will begin passenger service in the mid-2020s,” Boom said in a statement, adding: “Future customers include Virgin Group and Japan Airlines, which have pre-ordered a combined 30 Overture airliners.”

Boom has yet to announce the selection of an engine for the three-engined Overture, and has acknowledged the difficulty in finding an available turbine engine that can be adapted to provide the supersonic performance required while meeting airport noise limits.

Graham Warwick