(Reuters) -More than 3,200 union members who assemble Boeing's fighter jets in the St. Louis area and Illinois went on strike on Monday after rejecting a second contract offer the previous day.

Boeing Defense said it was ready for the work stoppage and it will implement a contingency plan that uses non-labor workers.

According to the company, the rejected four-year contract would have raised the average wage by roughly 40% and included a 20% general wage increase and a $5,000 ratification bonus. It also included increasing periodic raises, more vacation time and sick leave.

"We're disappointed our employees in St. Louis rejected an offer that featured 40% average wage growth," Dan Gillian, Boeing vice president and general manager of the St. Louis facilities, said in a statement.

The offer was largely the same as the first offer that was overwhelmingly rejected one week earlier.

Members of the International Association of Machinists and Aerospace Workers' District 837 "deserve a contract that reflects their skill, dedication, and the critical role they play in our nation's defense," District 837 head Tom Boelling said in a statement.

Boeing CEO Kelly Ortberg downplayed the impact of a strike when talking with analysts on Tuesday about second-quarter earnings, noting that the company had weathered a seven-week strike last year by District 751 members, who build commercial jets in the Northwest and number 33,000.

"I wouldn't worry too much about the implications of the strike. We'll manage our way through that," he said.

District 837 workers assemble Boeing's F-15 and F/A-18 fighters, the T-7 trainer, and the MQ-25, an aerial refueling drone being developed for the U.S. Navy.

Boeing's defense division is expanding manufacturing facilities in the St. Louis area for the new U.S. Air Force fighter jet, the F-47A, after it won the contract this year.

District 751's strike ended with approval of a four-year contract that included a 38% wage increase.