Employees want better wages, pensions and healthcare from Boeing, the aerospace giant that has faced a number of safety scandals this year.
ADVERTISEMENTAround 33,000 workers at Boeing laid down their tools from midnight Pacific Time on Friday (8AM CEST), after voting to strike on Thursday.
The action comes as a fresh blow to the aircraft maker, that is currently facing both financial and reputational challenges.
The International Association of Machinists and Aerospace Workers said that its members had rejected a contract that would have raised pay by 25% over four years.
The union added that 94.6% had voted in favour of rejecting the contract, while 96% had voted in favour of the strike. Only a two-thirds majority was required for the strike to go ahead.
Union leaders had initially advised members to accept the proposed contract earlier this week.
As well as raising wages, it would have kept the construction of Boeing's next commercial aircraft in the Seattle area, provided that the project started before the end of the contract.
Workers are, however, angry about stagnant wages and concessions they have made since 2008 on pensions and healthcare.
They argue that perks have been surrendered over the years to prevent the company from moving jobs elsewhere.
"This is about respect, this is about addressing the past, and this is about fighting for our future," IAM District 751 President Jon Holden said while announcing the vote.
Boeing stressed that it was "ready to get back to the table to reach a new agreement" after the vote.
"The message was clear that the tentative agreement we reached with IAM leadership was not acceptable to the members. We remain committed to resetting our relationship with our employees and the union," the company said in a statement.
The strike ignores a last-minute plea from Boeing's new chief executive Kelly Ortberg, who was appointed last month.
Just before the vote, he warned workers that the strike would put the firm's "recovery in jeopardy", adding that "no one wins" in a walkout.
Very little has gone right for Boeing this year, from a panel blowing out in one of its passenger jets in January to NASA leaving two astronauts in space rather than sending them home on a problem-plagued Boeing spacecraft.
As long as the strike continues, Boeing will be deprived of much-needed cash that it gets from delivering new planes to airlines.
