More than 3,200 unionised Boeing workers strike at company's facilities against the new four-year contract.
The rejected four-year contract would have raised the average wage by roughly 40% and included a 20% general wage increase, according to the company.
IAM District 837 seeking to have a contract offer that reflects their hardwork, dedication and skill.
More than 3,200 unionised workers at Boeing facilities in Missouri and Illinois went on a strike at midnight Monday after failing to reach an agreement on a labour contract.
Boeing Defense said it was ready for the work stoppage and it will implement a contingency plan that uses non-labour workers.
"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.
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.
"IAM District 837 members build the aircraft and defence systems that keep our country safe. They deserve nothing less than a contract that keeps their families secure and recognises their unmatched expertise," IAM Midwest Territory General Vice President Sam Cicinelli said in a statement on Sunday.
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, reported Reuters.
The offer was largely the same as the first offer that was overwhelmingly rejected one week earlier, they added.
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 refuelling drone being developed for the US Navy.
Boeing's defence 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.
Late last month, the union announced that members of District 837 had "overwhelmingly voted to reject" Boeing's contract offer. The previous contract expired just before midnight on July 27.
