United Airlines buys $1B in electric aircraft from Silicon Valley startup

United Airlines (Nasdaq: UAL) is buying $1 billion worth of aircraft from a Silicon Valley startup that's developing small electric aerial vehicles that could one day bring "air taxis" to cities. Palo Alto-based Archer announced Wednesday that United plans to purchase 200 of its Archer eVTOL (electric vertical takeoff and landing) aircraft, valued at $1 billion. The Chicago-based airline — the largest passenger carrier at Denver International Airport — said it has the option to purchase an…