Boeing Co mechanics and engineers are working round the clock to make sure the new 787 Dreamliner, already two years behind schedule, makes its first test flight before the end of June.

The revolutionary, carbon composite aircraft -- assembled near Seattle from parts made around the world -- was supposed to fly in the summer of 2007, but has been pushed back four times by production snafus and delayed by a two-month mechanics' strike.

Boeing has reshuffled its schedule a number of times, but now looks to be on track to finally get the first 787 into the air sometime in the second quarter, as promised in December. If the tests go to plan, it will start delivering planes to airlines in the first quarter of 2010.

That is crucial to Boeing, which gets the bulk of the payment for a plane when it is delivered. Despite a few cancellations this year, Boeing has 878 Dreamliners on order, worth about $140 billion at list prices.

Boeing gave reporters a rare glimpse of the 787 production plant on Thursday, where it has eight of the aircraft in various stages of assembly.

Two are for ground tests only, and will never fly. The other six will form the fleet of planes that Boeing will fly for a total of 3,500 hours over an eight- or nine-month period to meet the U.S. Federal Aviation Administration's stringent certification.

The first 787 scheduled to fly is the same one that was rolled out in front of thousands of cheering Boeing employees in July 2007, then hastily pulled back for more work.

It is now stationed in Boeing's paint hangar, where it has already been touched up and had its fuel tanks washed out in preparation for the first flight. Mechanics swarmed about the plane on Thursday, making checks and adjustments inside and out.

Inside Boeing's massive wide-body plant at Everett, Washington, some 30 miles north of Seattle, there is a nose-to-tail line of 787s in various stages of construction, set to become test flight planes.

Workers glide by on tricycles, making their way across one the world's biggest buildings, which Boeing says could fit 911 basketball courts.

Up above the production line, Boeing has set up its 'production integration center' -- a cross between NASA's mission control and a railroad dispatch office -- where up to 30 people monitor the production and shipment of 787 parts from around the world.

Much of the plane's structure is manufactured in Japan and Italy and flown in enlarged 747 jumbos to be joined in South Carolina, then flown onto Everett for final assembly. A 40-foot (12-meter) long screen monitors potential problems, tracks flights delivering structures and even feeds real-time data on earthquakes around the world that might affect production.

The center is staffed 24 hours a day, six days a week, ensuring continuity between Japan, Italy and the United States. The only lull in the work week comes between late Saturday afternoon, when work at U.S. plants slows, and Sunday afternoon, when production in Japan resumes.

Boeing still won't say exactly when the first test flight will take place, but industry-watchers reckon it will happen around the middle of June.

If the tests go well, Japan's All Nippon Airways <9202.T> is set to take delivery of the first production 787 in the first quarter of next year, nearly two years after the initial delivery date of May 2008.

(Reporting by Bill Rigby; Editing by Richard Chang)