Boeing Co. is facing its first strike in nearly 30 years at its defense facilities near St. Louis, United States, after union members rejected a revised contract offer.

Approximately 3,200 machinists are expected to begin striking at midnight, following a vote against a proposal that included a 20% wage increase and higher retirement contributions.

The last strike by the union occurred in 1996 and lasted 99 days.

“IAM District 837 members have spoken loud and clear, they deserve a contract that reflects their skill, dedication, and the critical role they play in our nation’s defense,” Tom Boelling, the union local’s top official, said in a statement.

The strike is expected to intensify financial strain on Boeing’s defense and space division, which accounted for roughly 30% of the company’s revenue in the second quarter.

“We are prepared for a strike and have fully implemented our contingency plan to ensure our non-striking workforce can continue supporting our customers,” Dan Gillian, a Boeing vice president and senior St. Louis site executive, said in a statement.

Union members produce fighter jets like the F-15 and the T-7 training aircraft, as well as missiles, munitions, and components for Boeing’s 777X commercial jets.

Boeing tailored its latest offer to address concerns raised by members of the International Association of Machinists and Aerospace Workers Local 837 after they rejected an initial proposal the previous week, spokesperson Gillian told reporters on July 31.

Both proposals were the most generous Boeing has ever made to the district’s members, with average wages set to rise from $75,000 to $102,600 for IAM 837 workers.

The company also dropped a contentious scheduling plan and revised its 401(k) offer to provide the full contribution increase immediately, rather than phasing it in over three years.