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 threat of labor unrest follows last year’s two-month strike by a separate machinists union in Seattle, which disrupted Boeing’s ( BA) commercial jet production and prompted the company to raise $24 billion through an equity offering to offset the financial toll.

Several thousand Boeing union workers at three St. Louis-area plants who build fighter jets are planning to go on strike after rejecting a proposed contract.