Another contract proposal has been rejected by Boeing workers who now have been on strike for nearly six weeks from three Midwest plants where military aircraft and weapons are developed.

The vote on Friday refusing the latest proposal sends the workers back to the picket lines, according to the union representing the 3,200 striking workers who build fighter jets, weapons systems and the U.S. Navy’s first carrier-based unmanned aircraft. Fifty-seven percent of members voted against the proposal, the union said.

“Boeing’s modified offer did not include a sufficient signing bonus relative to what other Boeing workers have received, or a raise in 401(k) benefits,” the International Association of Machinists and Aerospace Workers District 837 said in a statement.

“We’re disappointed our employees have rejected a 5-year offer, including 45% average wage growth," said Dan Gillian, Boeing Air Dominance vice president and general manager, in an emailed statement. “We’ve made clear the overall economic framework of our offer will not change, but we have consistently adjusted the offer based on employee and union feedback to better address their concerns.”

Boeing said no further talks are scheduled.

"We will continue to execute our contingency plan, including hiring permanent replacement workers, as we maintain support for our customers,” Gillian said.

The strike, which began Aug. 4, is far smaller in scale than a walkout last year by 33,000 Boeing workers who assemble commercial jetliners. Still, the work stoppage has threatened to complicate the aerospace company’s progress in regaining its financial footing.

Boeing’s Defense, Space & Security business accounts for more than one-third of the company’s revenue.

Boeing Co., based in Arlington, Virginia, employs more than 170,000 workers in the U.S. and more than 65 other countries.