The U.S. Air Force is moving forward with plans to purchase a new F-15 Eagle fighter jet, initiating its first fourth-generation fighter program in more than 20 years.

In a presolicitation notice recently posted on the government's acquisition and awards website, beta.sam.gov, the service announced it will sole-source two contracts, one for the F-15EX and the other for its F110 engines.

The Air Force Life Cycle Management Center "intends to award a sole source indefinite delivery/indefinite quantity (ID/IQ) contract to The Boeing Company for a refresh to the F-15C/D fleet and augment the F-15E fleet," one solicitation reads. The Defense Department expects a response from Boeing by Feb. 7.

The center also intends to award another ID/IQ contract to General Electric Aviation "to provide F110 propulsion systems to meet the F-15EX weapons system requirement," according to the second notice, which has the same response due date.

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The Air Force wants at least eight new F-15 "fourth-plus" variants in its inventory. Boeing has said the fighter will be equipped with better avionics and radars and could carry more than two dozen air-to-air missiles.

In December, Congress signed off on the plan, but with a caveat: The Air Force requested $1.05 billion for eight aircraft, but lawmakers are limiting the buy to just two at first, according to the fiscal 2020 defense appropriations bill.

"Of the funds provided in Aircraft Procurement/Air Force for the remaining six F-15EX aircraft, no more than $64,800,000 for long-lead materials may be obligated until the Secretary of the Air Force submits a report" regarding the program's strategy and future schedule, the bill stated.

The Air Force expects to keep a well-rounded mix of fourth- and fifth-generation aircraft through the 2030s, including the F-35A Joint Strike Fighter, F-16 Fighting Falcon, F-22 Raptor, A-10 Thunderbolt II and F-15 Eagle/Strike Eagle, officials have said.

Then-Air Force Secretary Heather Wilson told reporters at the annual Air Force Association Air Warfare Symposium last year that the service needed to boost its fighter inventory to stop the average age its aircraft inventory from increasing.

The service has estimated it needs to buy 72 new aircraft per year to replace those old planes; officials just didn't expect to do so with the F-15EX.

"Our budget proposal that we initially submitted did not include additional fourth-generation aircraft," Wilson said Feb. 28, 2019, adding that supplemental decisions must support the "overall presidential budget."

The Air Force has been on a quest to replace its aging F-15C models. Officials in 2017 voiced concerns about the aircraft's longevity.

"We are already having serious problems with that airframe, with metal fatigue within the longerons on the side of the aircraft," Wilson said during a forum last May.

Senior defense officials with the Cost Assessment and Program Evaluation office told reporters that they arrived at the Boeing-made F-15EX decision because the aircraft would help keep a "robust industrial base" and provide "a higher-capacity" combination alongside Lockheed Martin's F-35.

"One of the considerations was the diversity of the industrial base," a senior defense official said at the Pentagon on March 22, 2019.

"Maintaining a diverse industrial base is in the best interest of the Department of Defense. The more diversity, the more competition ... and the better prices we have," the official said.

-- Oriana Pawlyk can be reached at oriana.pawlyk@military.com. Follow her on Twitter at @Oriana0214.

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