The planemaker on Monday presented what it called its " final and best offer " — but union bosses rejected it and criticized Boeing for a "blatant show of disrespect."
Some 32,000 Boeing employees are members of the International Association of Machinists and Aerospace Workers (IAM) and have been on strike since September 13.
Nearly 95% of them voted against Boeing's initial offer of a 25% pay rise.
That was despite union leadership recommending workers vote in favor of the deal after negotiations continued into the early hours.
Boeing upped its proposal on Monday to a 30% pay rise plus a doubling of the contract-signing bonus to $6,000.
In a statement , IAM District 751, which represents workers in the Seattle area, slammed the planemaker for sending the offer to members and the media while union leaders were still reviewing the details.
Boeing was also seen promoting the offer as a sponsored ad on Instagram stories.
"These direct dealing tactics are a huge mistake, damage the negotiation process, and attempt to go around and bypass your Union negotiating committee," the union negotiating committee said.
"This tactic is a blatant show of disrespect to you — our members —and the bargaining process," they added.
In a statement shared with Business Insider, Boeing said it has "bargained in good faith with the IAM since formal negotiations began in March."
"After an unsuccessful federal mediation last week, we presented a best and final offer that made significant improvements and addresses feedback from the union and our employees," it added.
"We first presented the offer to the union and then transparently shared the details with our employees."
IAM District 751 President Jon Holden told The Seattle Times the latest offer has "some positives," but criticized Boeing for not engaging in discussions about it.
He also added that the union wouldn't logistically be able to produce a vote by the Friday deadline.
The IAM looks positioned to hold out for a better offer. Members have been issued with debit cards that will receive $250 a week when the strike enters its third week, the BBC reported.
38-year-old Ryan Roberson, who works in Boeing's final assembly division, told the outlet that $250 "will buy a lot of Top Ramen."
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