The workers' first strike since 2008 comes as the plane maker is under heavy scrutiny from US regulators and customers after a door panel blew off a 737 MAX mid-air in January.

The mounting crises battered Boeing's stock and sparked a leadership upheaval. Boeing shares fell 4 percent in US premarket trading on Friday.

The stock has fallen nearly 38 percent since the start of 2024.

Kelly Ortberg was brought in as Boeing's new CEO just weeks ago to restore faith in the plane maker and had proposed a deal that included a pay rise of 25 percent over four years, far lower than the 40 percent workers had demanded.

Roughly 30,000 members of the International Association of Machinists and Aerospace Workers (IAM) who produce Boeing's 737 MAX and other jets in the Seattle and Portland areas voted on their first full contract in 16 years, with 94.6 percent rejecting it and 96 percent favoring a strike in a two-part ballot.

"This is about respect, this is about addressing the past, and this is about fighting for our future," said Jon Holden, who headed the negotiations for Boeing's largest union, before announcing the vote result on Thursday evening.

"We strike at midnight," said the union leader, who had agreed to the just-defeated deal, as members in the union hall cheered and chanted: "Strike! Strike! Strike!"

A long strike could badly hit Boeing's finances, which are already groaning due to a US$60 billion debt pile.

"We remain committed to resetting our relationship with our employees and the union, and we are ready to get back to the table to reach a new agreement," Boeing said in a statement on Thursday.

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

"The key question now is on the duration of the strike, given the gap between the proposed wage increase and union members’ request," Jefferies analyst Chloe Lemarie said in a note, adding that a long strike represented a key risk for 737 MAX production levels.

Although IAM leadership last Sunday recommended its members accept the contract, many workers responded angrily, arguing for the original demand and lamenting the loss of an annual bonus.

"We're going to get back to the table as quickly as we can," Holden told reporters, without saying how long he thought the strike would last or when talks would resume.

"This is something that we take one day at a time, one week at a time.”

A strike presents Boeing with multiple challenges: It will need to decide how to respond at the bargaining table after saying it had offered everything it could; and it must also find a way to secure factories full of valuable, partly built planes.

Workers have been protesting all week in Boeing’s Seattle factories that assemble the MAX, 777 and 767 jets.

Shortly after midnight, striking workers started to gather outside the entrances of the Seattle factories, many waving placards that read “On Strike Against Boeing” and passing drivers honking their horns in support.

“I’m willing to strike for two months or even longer. Let’s go as long as it takes to get what we deserve,” said James Mann, a 26-year-old who works in Boeing’s wings division.

A prolonged strike would also weigh on airlines that depend on the plane maker’s jets as well as suppliers that manufacture parts and components for Boeing aircraft.

Air India CEO Campbell Wilson said on Friday that 737 MAX deliveries to his airline appeared to be "delayed a little bit" even before the strike announcement, due to regulatory scrutiny after the Alaska Airlines mid-air blowout and supply chain issues affecting the broader industry.

"There's nothing official yet, but I think the indication is, or the expectation is, that it's going to be a little bit later," he said in an interview in Sydney.

According to a pre-vote note from TD Cowen, a 50-day strike could cost Boeing $3 billion-3.5 billion of cash flow.

The last strike in 2008 shuttered Boeing’s plants for 52 days and hit revenue by an estimated $100 million per day.

S&P Global Ratings said that an extended strike could delay the plane maker's recovery and hurt its overall rating.

Both S&P and Moody's rate Boeing one notch above junk status.

The White House did not immediately respond to a request for comment.