NEW YORK — Boeing workers who build fighter jets are planning to go on strike Monday at midnight.

About 3,200 workers at Boeing facilities in St. Louis; St. Charles, Mo.; and Mascoutah, Ill., voted to reject a modified four-year labor agreement with Boeing, the International Association of Machinists and Aerospace Workers union said Sunday.

“IAM District 837 members build the aircraft and defense systems that keep our country safe,” said Sam Cicinelli, Midwest territory general vice president for the union, in a statement. “They deserve nothing less than a contract that keeps their families secure and recognizes their unmatched expertise.”

The vote followed members’ rejection last week of an earlier proposal from the troubled aerospace giant, which had included a 20% wage increase over four years.

At the time, union leaders had recommended approving the offer, calling it a “landmark agreement” and saying the offer would improve medical, pension and overtime benefits.

Then there was a cooling-off period of a week, followed by the union members rejecting Boeing’s latest proposal.

“We’re disappointed our employees rejected an offer that featured 40% average wage growth and resolved their primary issue on alternative work schedules,” said Dan Gillian, Boeing Air Dominance vice president and general manager, and senior St. Louis site executive. “We are prepared for a strike and have fully implemented our contingency plan to ensure our non-striking workforce can continue supporting our customers.”

Boeing has been struggling after two of its Boeing 737 Max airplanes crashed, one in Indonesia in 2018 and the other in Ethiopia in 2019, killing 346 people. In June, one of Boeing’s Dreamliner planes, operated by Air India, crashed, killing at least 260 people.

On Tuesday, Boeing had reported that its second-quarter revenue had improved and losses had narrowed. The company lost $611 million in the second quarter, compared to a loss of $1.44 billion during the same period last year.