Boeing on Monday announced an additional $450 million investment in Wisk Aero, expanding its bet on autonomous, electrified flight. The funding will help to fuel ambitious growth plans at Wisk, which is working to develop a self-flying electric air taxi.

“With this investment, we are reconfirming our belief in Wisk’s business and the importance of their work in pioneering all-electric, AI-driven, autonomous capability for the aerospace industry,” Marc Allen, Boeing’s chief strategy officer, said in a statement. Self-flying aircraft are “key to unlocking scale” in passenger and cargo flight, he added.

Boeing declined to share the size of its previous investment in Wisk, a joint venture of Boeing and Kitty Hawk, which is financed by Larry Page, who co-founded Google. A spokesperson for Wisk, which is based in Mountain View, California, declined to say how much total investment it had received so far but said Boeing had been the company’s largest investor since the joint venture was formed. Wisk employs about 360 people and plans to increase that workforce by about 70% during the next year.

The niche market for electric aircraft that take off like helicopters, fly like planes and can carry a handful of passengers short distances has attracted billions of dollars in investment in recent years, in the hope that the technology could supplement and supplant urban transportation and help airlines to offset their greenhouse gas emissions.

A handful of air taxi startups, including Archer Aviation, Joby Aviation, Lilium and Vertical Aerospace, went public last year and have a combined valuation of about $6 billion today.

American Airlines, a major Boeing customer, invested in Vertical Aerospace and has said it plans to buy up to 250 of that company’s aircraft. United Airlines, another major Boeing customer, invested in Archer and plans to buy up to 200 aircraft from it. Wisk sued Archer last year, accusing it of stealing trade secrets and infringing on its patents.

Wisk is developing its sixth-generation aircraft. If that autonomous vehicle is certified for flight, the company plans a fleet allowing it to carry out millions of annual zero-emission flights in nearly two dozen cities, it said. The Federal Aviation Administration has yet to approve any electric air taxi, autonomous or not, for flight, and experts say it could still be years before such aircraft are ready for public use.