LOS ANGELES — Wanted by the Pentagon: A muscular, outdoorsy specimen. Must be intelligent and, above all, self-driven.

When 20 hulking robotic vehicles face off next month in a rugged race across the Nevada desert, the winning machine (if any crosses the finish line) will blend the latest technological bling and the most smarts.

The military sponsors the race to speed the development of unmanned vehicles for combat. The project had an inauspicious start: Last year's inaugural contest ended soon after it began when the robots careened off course or abruptly stalled. One even got tangled in barbed wire.

Fast forward 18 months, and double the prize to $2 million.

Newcomers have joined a handful of last year's teams to form a motley mix of garage tinkerers, academia and corporations. All hope that their machines — fitted with the latest sensors, cameras and computers — have aged a generation since last year.

Teams have beefed up their vehicles' artificial intelligence through improved computer algorithms that will help them avoid pitfalls such as ditches and boulders strewn across the roughly 150-mile-long course. To get there, the robots must compete in a semifinal showdown that starts tomorrow.

This year's race shows signs of being extremely competitive. Some vehicles have logged hundreds of self-guided miles in the Southwest desert during summer practice runs. Several even tested on last year's course, which spanned the Mojave Desert between Barstow, Calif., and Primm, Nev.

The ramped-up preparation reflects the higher stakes for the so-called Grand Challenge. While sweetening the purse, organizers promise that the course, which this year loops from and to Primm, a casino town on the Nevada-California border, will be tougher and meaner.

Vehicles will have to drive on dirt and gravel, maneuver mountain switchbacks, squeeze through choke points and avoid manmade and natural obstacles.

The sponsor of the Grand Challenge is the research arm of the Pentagon known as the Defense Advanced Research Projects Agency, or DARPA. The Pentagon wants one-third of the military's ground vehicles to be unmanned by 2015.

DARPA Director Anthony Tether hopes that a robot will be able to traverse the course in less than 10 hours and snatch this year's prize. "It's going to be a long day out in the desert," Tether said.

Tomorrow, 40 teams and three alternates compete for a spot in the Oct. 8 race during the semifinals at the California Speedway in Fontana.

The vehicles must negotiate a two-mile stretch of the track using on-board computers, global positioning satellites and various lasers and radar. The top 20 performers will advance to the final starting line.

One of the favorites again this year is the Red Team from Carnegie Mellon University in Pittsburgh, led by robotics professor William "Red" Whittaker.

During last year's finals, Carnegie Mellon's converted Humvee, nicknamed Sandstorm, traveled the farthest — all of 7 1/2 miles — before breaking down. This year, the school entered an improved Sandstorm and a converted Hummer named Highlander. "I'm so hungry for race day," Whittaker said.

Among the newcomers: the Stanford Racing Team, whose modified Volkswagen Touareg, Stanley, recently drove 200 miles without interruption or human help in the Arizona desert.

The autonomous robotic vehicles use "drive-by-wire" technology, in which on-board computers control steering, braking and other movement.

If no one wins this year, the stakes will be raised again, to $4 million.