MILL CREEK, Wash. — Shannon Ryker is a third-generation employee of aerospace giant Boeing Co. She followed her grandfather into the huge plant in nearby Everett. And her father. And her Uncle Bob.

Her youngest sister worked at Boeing until she became pregnant. Both of Ryker's brothers-in-law and one of their dads work there. Her other sister's stepson has applied for a Boeing job.

So it wasn't easy for the 37-year-old mechanic to sit down in her crowded apartment here on a recent Sunday and write to Boeing management about her growing disappointment.

"Like my 86-year-old grandmother, I would like to tell my children and grandchildren that 'Boeing has been good to this family,'" Ryker wrote in an open letter that has since landed on company break-room tables and in co-workers' email in-boxes. But now, she said, "I no longer can hold my head high and say I am proud to work at Boeing."

At issue is the company's hunt for a site to build its newest airliner, the 777X. Ryker and other members of the International Assn. of Machinists and Aerospace Workers District 751 overwhelmingly voted last month to reject a contract that would have cut some pension plans and healthcare benefits but guaranteed the program would stay in the Pacific Northwest.

FULL COVERAGE: Building Boeing's 777X jetliner

Since the vote, Washington's largest private employer has been looking elsewhere for a site to build the plane, a potential move that threatens the state economy and the middle class Boeing helped create.

The company's decision reflects the hard realities of the industry and the latest skirmish in the fight for union survival. Boeing says the contract concessions are essential to compete financially with its longtime European rival Airbus, which plans to deliver its own new twin-aisle jetliner next year.

The two companies have been locked in a duopoly of the large jet market since the 1990s and are currently in a dead heat. Last year, Boeing delivered just 13 more aircraft than Airbus, a far cry from its 297-plane advantage in 1989.

Boeing Commercial Airplanes Chief Executive Raymond L. Conner laid out the stakes in a letter to workers before the Nov. 13 union vote on the 777X, an essential part of the company's long-term product strategy.

"What we want to avoid is that we become one of the companies that made decisions too late to remain competitive in the marketplace," he wrote.

Boeing gave other states until Tuesday to submit proposals to build the wide-body's latest generation. Within days of the union vote, California, Missouri and Texas made appeals to Boeing in an attempt to snag the program.

The company joins a long line of manufacturers and municipalities that have sought to wring concessions from unions that once negotiated comfortable pensions and wages.

After a bitter strike in 2008, the company shipped much of the work on its 787 Dreamliner to South Carolina, a right-to-work state. Seven years earlier, it moved its headquarters from Seattle to Chicago. Its Washington workforce is more than 83,000 strong, but there are fears that the company's future is elsewhere.

"If Boeing doesn't build the 777X here, this could be the start of a long, steady decline of the company's presence here," said Scott Hamilton, an aviation industry consultant who figures Boeing could be gone by 2030, based on backlogs and production rates.

"Sure it can happen," Hamilton said. "Thirty to 40 years ago, Southern California was the hub of commercial aerospace. Now, no [aerospace] company is based there."

Boeing was responsible for $70 billion of Washington's $76-billion aerospace industry in 2012. But unlike bankrupt Detroit, whose fortunes lived and died with autos, Puget Sound has diversified since the 1970s, when an enormous layoff called the "Boeing Bust" prompted a rueful billboard: "Will the last person leaving Seattle turn out the lights."

Washington has taken desperate measures to ensure that its flagship employer remains key to the economy. On Nov. 5, Gov. Jay Inslee announced that he was calling a special session of the Legislature to approve a massive package of tax breaks designed to keep the 777X in Washington. "These jobs are ours," the liberal Democrat said, "if we act now."

Less than a week later, state legislators passed the biggest corporate tax subsidy in U.S. history — $8.7 billion.

But the lawmakers' actions didn't cement the deal. The machinists needed to approve a new eight-year contract with the company, but they rejected it by a 2-1 ratio.