It wasn't a helicopter. And it wasn't an airplane. It was a cross between the two, with a curved hull, two small wings, and eight spinning rotors lined up across its nose and tail.

At the touch of button on a computer screen under a nearby tent, it stirred to life, rising up from a grassy slope on a ranch in central California and speeding toward some cattle grazing under a tree — who did not react in the slightest.

"It may look like a strange beast, but it will change the way transportation happens," said Marcus Leng, the Canadian inventor who designed this aircraft, which he named BlackFly.

BlackFly is what is often called a flying car. Engineers and entrepreneurs like Mr. Leng have spent more than a decade nurturing this new breed of aircraft, electric vehicles that can take off and land without a runway.

They believe these vehicles will be cheaper and safer than helicopters, providing practically anyone with the means of speeding above crowded streets.

"Our dream is to free the world from traffic," said Sebastian Thrun, another engineer at the heart of this movement.

That dream, most experts agree, is a long way from reality. But the idea is gathering steam. Dozens of companies are now building these aircraft, and three recently agreed to go public in deals that value them as high as $6 billion. For years, people like Mr. Leng and Mr. Thrun have kept their prototypes hidden from the rest of the world — few people have seen them, much less flown in them — but they are now beginning to lift the curtain.