Northrop Grumman Corp. and the Boeing Co. submitted final bids for the Air Force’s tanker contract late Thursday, which could be worth as much as $100 billion.

The first phase of the KC-X contract, worth $40 billion, will replace the Air Force’s fleet of KC-135 refueling jets with 179 of the winner’s newer models. The average age of the Air forces fleet of tankers is close to 50 years old.

Los Angeles-based Northrop’s partner in the project is the European Aeronautic Defense and Space Co., and has Northrop modifying an Airbus A330 for the project, which Northrop calls the KC-30.

Boeing’s entry is called the KC-767 Advanced Tanker and is modified from the Chicago-based company’s 767-200 long-range freighter.

The companies expect the final award to be announced by the end of January.

The current KC-X deal is the first of an expected three-phase deal that would call for more than 500 planes and would be worth an estimated $100 billion.