Workers voted overwhelmingly to support the strike.
Boeing workers went on strike after they were overwhelmed rejected a tentative deal between union representatives and the aircraft manufacturer that included a 25 percent pay increase.
More than 30,000 workers in Seattle and Portland stopped work from midnight Pacific time (07:00 GMT) on Friday.
The strike is another setback for the company, which is already facing mounting financial losses.
The company is also struggling to restore its reputation after a series of safety problems, including two fatal crashes.
Strike would be a major blow to Boeing new CEO Kelly Ortbergwho was appointed last month with a mission to turn the business around.
Nearly 95% of union members — who make planes including the 737 Max and 777 — voted against the pay deal.
Of those who voted, 96% supported strike action until a new deal is reached.
“Our members spoke out loud and clear tonight,” said Jon Holden, president of the International Association of Machinists and Aerospace Workers (IAM) District 751.
“We attacked at midnight.”
“The message made clear that the interim agreement we reached with IAM leadership was not acceptable to members,” Boeing said in a statement.
“We remain committed to rebuilding relationships with employees and unions, and we are ready to return to the negotiating table to reach a new agreement.”
The strike is a blow to Boeing and an embarrassment to Mr Ortberg, who has offered a last plea to workers ahead of the vote, warning that the strike would put the company’s “recovery at risk”.
The question now is how long the situation will last. Boeing appears ready to return to the negotiating table.
But there is a clear breakdown of trust between management and the workforce — and between the workforce and the union leadership, who have said this is the best contract they have ever negotiated and have urge members to accept the agreement.
In addition to a 25 percent pay raise over four years, the tentative deal that workers rejected included a commitment from Boeing to build its next commercial plane in the Seattle area if the project starts within the contract term.
The union initially sought some improvements to the workers’ benefits package, including a 40 percent pay increase.
In reality, a quick solution is unlikely unless Boeing surrenders.
Analysts say a prolonged shutdown could cost the company and its suppliers billions of dollars.
The current contract between Boeing and the unions was signed in 2008 after an eight-week strike.
The strike is costing the company around $1.5bn (£1.14bn) a month, according to credit ratings firm Moody’s.
In 2014, the two sides agreed to extend the deal, which expired at midnight Thursday.
“There’s never a good time to strike, at least from a management perspective, and the current situation makes it even more difficult,” said Greg Waldron, Asia managing editor of aviation news website FlightGlobal.
“However, it will be important to see how long the strike lasts. Executives with orders for the 737 Max will be watching this closely,” Waldron added.
In July, Boeing agreed to plead guilty to fraud and pay a criminal fine of nearly $244 million in connection with two fatal crashes of its 737 Max aircraft more than five years ago.
The airline is also facing other lawsuits and investigations following the explosion of a door stopper on a new Alaska Airlines plane in January.
In addition to suffering mounting financial losses, the aircraft maker has also had to slow down its assembly lines due to production limits on the 737 Max aircraft imposed by the US Federal Aviation Administration.
