United Airlines Holdings Inc. UAL 2.35%▲ has paid a $10 million deposit for 100 electric flying taxis, a sign that the airline is growing more confident in the nascent technology. United and a regional airline it partners with last year invested in Archer Aviation Inc. ACHR -3.20%▼ and struck a preliminary agreement to buy up to 200 of the flying taxis that the San Francisco Bay Area-based company is developing. Other airlines and leasing companies have announced their own investments in flying-taxi startups and preliminary orders. But the aircraft haven’t yet been approved by regulators to fly passengers, and customers generally haven’t had to put down cash. The aircraft being developed by Archer and its rivals take off and land vertically like helicopters. Airlines hope that they will be able to zip customers around cities, whisking them over congested highways to and from hub airports while reducing carbon emissions […]