Boeing Co. expects more than 3,200 union workers at three St. Louis-area plants that produce U.S. fighter jets to strike after they rejected a proposed contract Sunday that included a 20% wage increase over four years.

The International Machinists and Aerospace Workers union said the vote by District 837 members was overwhelmingly against the proposed contract. The existing contract was to expire at 11:59 p.m. Central time Sunday, but the union said a “cooling off” period would keep a strike from beginning for another week, until Aug. 4.

Union leaders had recommended approving the offer, calling it a “landmark” agreement when it was announced last week. Organizers said then that the offer would improve medical, pension and overtime benefits in addition to pay.

The vote came two days before Boeing planned to announce its second quarter earnings, after saying earlier this month that it had delivered 150 commercial airliners and 36 military aircraft and helicopters during the quarter, up from 130 and 26 during the first quarter. Its stock closed Friday at $233.06 a share, up $1.79.

The union did not say specifically why members rejected the contract, only that it “fell short of addressing the priorities and sacrifices” of the union’s workers. Last fall, Boeing offered a general wage increase of 38% over four years to end a 53-day strike by 33,000 aircraft workers producing passenger aircraft.

Dan Gillan, general manager and senior Boeing executive in St. Louis, said in a statement that the company is “focused on preparing for a strike.” He described the proposal as “the richest contract offer” ever presented to the St. Louis union.

“No talks are scheduled with the union,” said Gillan, who is also vice president for Boeing Air Dominance, the division for the production of several military jets, including the U.S. Navy’s Super Hornet, as well as the Air Force’s Red Hawk training aircraft.