Personal flying cars remain a dream mostly relegated to science fiction, but the introduction of "air taxis" above our heads is starting to look far more likely—for better or worse. Last week, The Wall Street Journal reported United Airlines is moving forward with an initial $10 million down payment on 100 airborne commuter vehicles from Archer Aviation. The investment is only a small portion of last year's $1 billion partnership announcement between the major airliner and much buzzed-about startup, but it's a major milestone within a burgeoning industry, one that indicates the new transportation option may be much more than mere vaporware.

"There's a lot of talk about orders in the industry, a lot of talk about paper commitments, but this is the first real cash commitment," Archer Aviation CEO Adam Goldstein said in a statement.