United Airlines Holdings Inc. and regional carrier Mesa Air Group Inc. each agreed to buy as many as 100 small electric-powered planes from a Swedish startup — if the aspiring manufacturer can develop the aircraft. The U.S. airlines and Breakthrough Energy Ventures also will take an equity stake in Heart Aerospace , the companies said in a statement Tuesday. The investment amounts to $35 million, Heart Chief Executive Officer Anders Forslund said in an interview before the announcement. The company is targeting a 2026 debut for its proposed 19-seater, the ES-19. The deal is part of United’s push to eliminate its carbon emissions by 2050 and thereby reduce its impact on climate change. Separately, the carrier’s United Airlines Ventures fund has also invested in Archer Aviation and made a conditional purchase of the aircraft that startup is developing, a four-seat electric air taxi with vertical takeoff and landing capability. […]