Boeing Co. delivered a final contract offer to its union machinists Thursday that it said would boost pay by 11 percent on average over three years, the latest effort by the airplane maker to avoid a strike that could cost it millions of dollars a day.

Chicago-based Boeing said the proposed contract was its “best and final” offer and would increase compensation for the more than 27,000 union workers in Washington state, Kansas and Oregon. It includes a $2,500 bonus for workers if the contract is ratified by next Wednesday – the day the current contract expires and a union vote is scheduled.

The company said it had withdrawn certain proposals that drew concern from the union, including plans to cut early retiree medical coverage and create a new defined-contribution retirement program for future employees.

Union leaders and members have a few days to scour the 300-page proposal.

The two sides have been engaged in round-the-clock negotiations at a Seattle airport hotel since Aug. 21. The last round of negotiations, in 2005, led to a strike.