What happened

Delta Air Lines (DAL -1.31%) will invest at least $60 million in Joby Aviation (JOBY -2.88%) as part of a broader plan to partner with the air taxi start-up. Shares of Joby opened up as much as 15% on the news, which was viewed as an endorsement of the company's business plan and technology.

So what

Joby is one of a number of companies to come public over the last 18 months focused on developing electric airplanes capable of vertical takeoffs and landings, or eVTOLs. The aircraft are designed to complement the jumbo jets that take passengers across continents by offering quick hops across a metropolitan area.

The planes are still experimental, but Delta has apparently seen enough in Joby to be a believer. On Tuesday, the airline giant announced an initial investment of $60 million in the company that could grow to $200 million over time, should Joby meet certain milestones.

Delta intends to integrate Joby's eVTOL service into its existing flight network in New York and Los Angeles, expanding to San Francisco, Miami, and London over time.

"This is a groundbreaking opportunity for Delta to deliver a time-saving, uniquely premium home-to-airport solution for customers in key markets we've been investing and innovating in for many years," Delta CEO Ed Bastian said in a statement.

Joby hopes to launch service in 2024, subject to regulatory approval.

Now what

There is currently a land grab of sorts going on among eVTOL companies. Joby now counts Delta, along with Toyota Motor and the venture arm of JetBlue Airways, as investors. Rival Archer Aviation is aligned with United Airlines Holdings, and Vertical Aerospace has received funding and orders from American Airlines Group.

There is seemingly a real market for these electric mini-planes that would be able to efficiently bypass road congestion and link major airports to outer suburbs. However, with so many players chasing a still-uncertain market, none of these stocks are a sure thing.

Joby's deal with Delta won't mean much if the company can't secure regulatory approval and figure out how to mass-produce its aircraft, but it is a massive step toward making sure there is a market for its planes if and when it does initiate commercial service.