SEATTLE (Reuters) - Boeing's U.S. West Coast factory workers will walk off the job after 96% voted on Thursday in favor of a strike, halting production of the planemaker's strongest-selling jet as it wrestles with chronic output delays and mounting debt.

The workers' first strike since 2008 will start at midnight Pacific time on Friday (0700 GMT), just weeks after new CEO Kelly Ortberg was brought on in August to restore faith in the planemaker after a door panel blew off a near-new 737 MAX jet in mid-air in January.

Roughly 30,000 workers who produce Boeing's 737 MAX, and other jets in the Seattle and Portland areas were voting on their first full contract in 16 years.

Under complicated rules set by the International Association of Machinists and Aerospace Workers (IAM), at least two-thirds of the unionized workers had to vote in favor of a strike for a stoppage to begin and the contract to be rejected.

Boeing workers voted 96% in favor of striking and 94.6% to reject the agreement.

The deal included a general wage increase of 25%, a $3,000 signing bonus and a pledge to build Boeing's next commercial jet in the Seattle area, provided the program is launched within the four years of the contract.

Although IAM leadership recommended last Sunday that its members accept the contract, many workers had responded angrily, arguing for the originally demanded 40% pay rise and lamenting the loss of an annual bonus.

On Wednesday, Ortberg sent a letter to workers, urging them to approve the deal.

Some workers were already preparing for picket lines that day, with one union member leaving a Wednesday meeting carrying a placard under her arm that read: "On Strike Against Boeing."