(Reuters) -U.S. planemaker Boeing sent a new contract offer on Thursday to the union representing roughly 3,200 factory workers at Boeing's defense division, mostly in the St. Louis area.

It contains some minor compensation changes that would benefit senior union members, the company said. The offer also keeps current overtime policies, which Boeing had proposed modifying in the last contract offer. 

If the contract is not passed by Sunday, a $5,000 lump sum bonus will not be offered again, according to Boeing. 

Members of the International Association of Machinists and Aerospace Workers overwhelmingly rejected a contract offer from Boeing on Sunday. The offer sent to members of IAM District 837 included a 20% general wage increase over four years, the bonus, and more vacation time and sick leave.

That offer was insufficient, the IAM union said at the time. 

"We have carefully considered and responded to the feedback from our employees and the union," Dan Gillian, a Boeing vice president, said in a statement.

He called the contract "the richest we've ever proposed for IAM 837."

The IAM declined to comment. 

The workers assemble Boeing's fighter jets 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, the F-47, after it won the contract this year.

The Next Generation Air Dominance 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.