(Reuters) -Union members who assemble Boeing's fighter jets in the St. Louis area have "overwhelmingly voted" to reject the company's contract offer on Sunday, with the company now preparing for an imminent strike.

Boeing's proposal, which was sent on Tuesday to more than 3,200 members of the International Association of Machinists and Aerospace Workers (IAM) District 837, included a 20% general wage increase over four years and a $5,000 ratification bonus, as well as more vacation time and sick leave.

"The proposal from Boeing Defense fell short of addressing the priorities and sacrifices of the skilled IAM Union workforce," the IAM union said.

Dan Gillian, Boeing's Air Dominance vice president, general manager and senior St. Louis Site executive said in an emailed statement that it is disappointed Boeing employees voted down "the richest contract offer we've ever presented to IAM 837 which addressed all their stated priorities."

"We've activated our contingency plan and are focused on preparing for a strike. No talks are scheduled with the union," Gillian added.

The current contract expires on Sunday following which there is a seven-day cooling off period before a strike would begin, the union added.

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

The Next Generation Air Dominance (NGAD) fighter jet program, initially conceived as a "family of systems" centered around a sixth-generation fighter jet, is meant to replace the F-22 Raptor.