The military’s newest eye in the sky just took flight.

Boeing’s Phantom Eye -- an unmanned, hydrogen-powered drone that can stay aloft for as much as four straight days -- completed its first autonomous flight June 1 at NASA's Dryden Flight Research Center at Edwards Air Force Base, Calif., following a series of test drives on the runway earlier this year.

It can also fly at 65,000 feet at approximately 150 knots and carry up to a 450-pound payload, thanks to a 150-foot wingspan and two 2.3-liter, four-cylinder engines that give it 300 horsepower -- 150 from each engine.

But the Boeing Phantom Eye isn’t just breaking ground from a defense perspective; it’s also a very “green” aircraft. With a liquid-hydrogen propulsion system, it has excellent fuel economy. And its only by-product is water.

The company says the altitude and endurance are clear advantages for reconnaissance and surveillance missions.

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"This day ushers in a new era ... where an unmanned aircraft will remain on station for days at a time providing critical information and services," said Darryl Davis, president of Boeing Phantom Works.

In April, a number of taxi tests were conducted to validate ground guidance, navigation and control, mission planning, pilot interface and operational procedures. On its first medium-speed taxi test in March, the Phantom Eye reached speeds of 30 knots.

On this inaugural, Phantom Eye took off at 6:22 a.m. PST for a 28-minute fledgling flight from its launch cart. The UAV climbed to an altitude of 4,080 feet and reached a cruising speed of 62 knots. It took on some damage when the landing gear broke digging into the lakebed after touchdown, however.

Having demonstrated handling and maneuverability, Boeing’s next test flight will involve flying Phantom Eye at a higher altitude.

This drone is the latest in a series of Boeing-funded rapid prototyping programs, which include Phantom Ray -- Phantom Eye’s big brother, which is arguably even more ambitious.

Fighter-jet sized, the Phantom Ray will be able to stay aloft for a whopping ten days and will be able to carry a payload of more than 2,000 pounds.

Big brother’s big first flight was April 27, 2011. On its second flight May 9, Phantom Ray reached an altitude of 7,500 feet and a speed of 178 knots. It had a clean landing after executing several maneuvers demonstrating basic air-worthiness and autonomy.

In addition to intelligence and surveillance, Phantom Ray will be useful in electronic attacks and suppression of enemy air defenses.

Both UAVs are creations of Boeing’s Phantom Works home to other cutting-edge autonomous technologies such as Echo Ranger.

Echo Ranger is also unmanned, but designed for autonomous work underwater rather than in the sky.  This unmanned submersible is 18.5 feet long, and in spite of weighing in at more than five tons, it can achieve speeds of eight knots. It can dive up to a depth of 10,000 feet and can travel as far as 80 miles without resurfacing.

On July 11, 2011, Echo Ranger achieved its first autonomous surface exit having dove to 40 feet and then later maneuvered to 400 feet on a pre-programmed course.

It can currently be configured for thirty-day missions and one of the objectives with Echo Ranger program is to achieve more than seventy days submerged.

In addition to undertaking operations abroad in enemy waters, Echo Ranger has great potential for patrolling and protecting U.S. coastline and harbors.

Originally designed back in 2001 for the oil and gas industry to take high-resolution sonar images of seabeds, Echo Ranger also has promising applications for green purposes: detecting environmental problems in the oceans and collecting water samples for analysis, for example.

Phantom Eye’s four days of unrefueled autonomous flight at 65,000 feet will introduce an unprecedented advantage in terms of data collection with persistent monitoring over large areas.

Unprecedented autonomous military machines in the air and the ocean don’t mean that machine-on-machine warfare is around the corner … but it will irrevocably change the game.

Ballet dancer turned defense specialist Allison Barrie has travelled around the world covering the military, terrorism, weapons advancements and life on the front line. You can reach her at wargames@foxnews.com or follow her on Twitter @Allison_Barrie.