Boeing Co said on Thursday it has opened the flight-test window for its 787 Dreamliner starting December 15, allaying concerns that the hotly anticipated aircraft -- already two years behind schedule -- will miss its latest target of taking to the skies before year end.

A test-flight date marks a huge step in the development of the revolutionary airplane, whose promise of increased fuel efficiency and comfort has attracted a record number of orders for a plane still in development.

Following the successful completion of static testing to validate the side-of-body modification, we have been working through a number of key preflight tests, Boeing said in a letter late on Thursday. With high-speed taxi testing scheduled in the coming days, we're on track for first flight soon thereafter.

The company said the date depends on final internal reviews, taxi test, and receiving the final experimental ticket from the U.S. Federal Aviation Administration.

In the last two years, Boeing has announced five embarrassing test-flight delays that resulted in delays to the delivery schedule as well.

The program suffered problems with suppliers, a 2008 labor strike and structural issues. In June, Boeing announced a test flight delay after it discovered a design flaw that required a side-of-body repair.

The flight from Paine Field to Boeing Field -- both in the Seattle area -- will take about three hours. The company has not disclosed who will be on hand to witness the event.

The Dreamliner concept is based on the use of composite materials, which make up 50 percent of the primary structure. The materials are much lighter than aluminum, which makes the skin of traditional commercial airplanes.

The lighter structure allows the plane to consume 20 percent less fuel than comparable aircraft. Fuel efficiency has great appeal to airlines, which have been battered in recent years by the volatile expense.

Additionally, composite materials do not fatigue and therefore require fewer maintenance checks, Boeing says. The company claims that a 787 costs 30 percent less to maintain than competing aircraft.

There are three versions of the plane with varying capacities and ranges in the Dreamliner family: the 787-8, 787-9, and the 787-3. The planes, which have been in development since the first part of the decade, have list prices between $105 million and $205.5 million.

(Editing by Lincoln Feast)