The Airborne Laser (ABL) was conceived to shoot down enemy ballistic missiles in the early stages of their flight.
In September, engineers fired the high-energy laser into a calorimeter aboard the aircraft. But this is the first time the beam has been fired along the full length of the 747.
"The team has now completed the two major milestones it hoped to accomplish in 2008, keeping ABL on track to conduct the missile shoot-down demonstration planned for next year," said Scott Fancher, vice president and general manager of Boeing Missile Defense Systems.
Onwards and upwards
The latest ground test was carried out by the US Missile Defence Agency at Edwards Air Force Base in California.
A laser beam travelled the length of the aircraft at one billion km/h (670 million mph).
It raced from the aft (back) section that houses the laser, through the beam control and fire control system, and out through the nose-mounted turret.
The high energy laser is fired from a turret in the aircraft's nose
When the laser beam emerged from the aircraft, it was captured by a diagnostic system which also provides simulated targets for the laser.
The next step is to carry out some long duration firings of the laser.
"Once we complete those tests, we will begin demonstrating the entire weapon system in flight," said Michael Rinn, Boeing vice president and programme director for the ABL.
The ABL is designed to illuminate an enemy missile with a laser tracking beam, while computers measure its distance and calculate its course and direction.
After acquiring and locking on to the target, a second, high-power laser fires a three-to-five-second burst from the turret in the 747's nose.
The beam heats up the pressurised fuel tank of the outbound missile and causes it to rupture, destroying the missile.