Boeing proposed lifting hourly wages for striking workers by 30 percent on Monday, sweetening its initial offer in an effort to end a 10-day stoppage that shuttered Seattle-area plants.
The aviation giant gave workers until Friday at midnight to ratify its "best and final offer."
The union, the International Association of Machinists and Aerospace Workers, said it was reviewing the proposal.
Some 33,000 IAM members from District 751 in the Pacific Northwest region walked off the job on September 13 after overwhelmingly voting down an earlier offer, effectively shutting down assembly plants for the 737 MAX and 777.
The 30 percent general wage increase improves on the 25 percent in the earlier offer, which was initially endorsed by IAM leaders before the rank-and-file workforce rejected it decisively.
Workers have sought a 40 percent wage increase, citing more than a decade of meager pay boosts that have taxed family budgets in a costly region of the United States during a period of consumer price inflation.
The new proposal also reinstates an annual bonus that had been removed in the earlier version.
Line workers had complained that the loss of the bonus meant that the earlier proposal amounted to less than the 25 percent wage hike advertised by the company.
The new proposal also doubles a ratification bonus to $6,000 and lifts the company's contribution to employees' 401K program. But the amended offer does not reinstate the pension, a demand of some workers.
The two sides undertook two days of mediation last week with assistance from government officials.
Boeing CEO Kelly Ortberg said ending the strike was "a top priority."
Boeing said the latest proposal would need to win ratification by 11:59 pm on Friday night, setting a tight timetable for IAM leadership to comment on the proposal and orchestrate a vote.
Surveys of line workers have shown general wages, the reinstatement of the bonus and the pension as priorities, the IAM has said.
Brian Bryant, president of the IAM international union, said the latest offer from Boeing "validates" the decision to strike.
"Employees knew Boeing executives could do better, and this shows the workers were right all along," Bryant said in a statement.
Boeing employee Mike Corsetti said he looked forward to studying the proposal in detail, saying, "it's closer but I'm not sure it's good enough."
The amended deal maintains other provisions, such as a pledge to build Boeing's next new airplane in the Pacific Northwest.
The strike has added to Boeing's woes as it faces heavy scrutiny from regulators due to safety problems.
Federal Aviation Administrator Mike Whitaker is scheduled to sit for two congressional hearings this week on the agency's oversight of Boeing.
Shares of Boeing rose two percent.
