WASHINGTON — Boeing on Monday bagged a $2.9 billion contact for the fourth lot of KC-46 tankers, raising the total to 52 aircraft on order.

The award, announced Sept. 10, covers the production of 18 KC-46s and other associated gear like spare engines and parts, support equipment and wing air refueling pod kits.

“We’re excited to partner with the Air Force on an aircraft that will provide its fleet unmatched capabilities and versatility,” said Mike Gibbons, Boeing KC-46A tanker vice president and program manager. “This is another big milestone for the team and we look forward to delivering this next-generation, multi-role tanker for years to come.”

Monday’s announcement follows a contract in December for the first foreign KC-46 order: a single tanker for Japan. The first two U.S. Air Force buys were finalized in August 2016 for a total of 19 aircraft, and a third order for another 15 KC-46s was added in January 2017. The service plans on buying 179 KC-46s over the course of the program.

Technical problems have kept Boeing from delivering the first KC-46, and the company has already missed an initial August 2017 deadline to deliver 18 fully-certified tankers to the service. However, it appears the company is getting ever closer to that milestone.

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Earlier this year, Boeing and the Air Force came to an agreement on the schedule for the first KC-46 delivery, currently slated for October.

Last week, Boeing disclosed that the KC-46 had received a supplemental type certificate from the Federal Aviation Administration — a series of tests that validate the aircraft’s refueling and avionics systems meet FAA requirements and a prerequisite for aircraft delivery.

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Gibbons called the event “one of the last major hurdles in advance of first delivery to the U.S. Air Force” in a Sept. 4 statement.