WASHINGTON — When it came time for the U.S. Defense Department to make a decision on which fourth-generation fighter to buy for the Air Force, industrial base considerations — and not acting Defense Secretary Patrick Shanahan — helped tip the scale in favor of Boeing’s F-15X, a senior defense official said Friday.

“There were other things on the table” besides the F-15X, said the official, who disclosed that the Pentagon’s Cost Assessment and Program Evaluation office drove the department’s decision to procure new fourth-gen planes to replace the Air Force’s aging F-15C/Ds.

But when CAPE, the Air Force and former Defense Secretary Jim Mattis finally agreed on the broad decision to more fourth-gen fighters, “the conversation then turned to: How are we going to maintain a robust industrial base?” the official said during a briefing with reporters.

The US Air Force doesn’t want F-15X. But it needs more fighter jets. The top two Air Force leaders made it clear that buying new fourth-generation planes was not their first choice.

“It really then turned into a conversation of, for the future of the Department of Defense, it’s going to be good to have multiple providers in the tactical aircraft portfolio, and that’s what led our way into the F-15X decision.”

This public acknowledgement of the behind-the-scenes discussions that led to the Air Force’s request for eight F-15Xs in its fiscal 2020 budget comes two days after the Defense Department’s inspector general announced it was investigating Shanahan. The IG is looking into allegations that Shanahan showed favoritism toward his former employer, Boeing, where he was employed for 30 years before being named deputy secretary of defense in 2017.

The Bloomberg report that initially broke the news of the F-15X procurement decision cited one unnamed source who stated that Shanahan influenced the process — something Shanahan has repeatedly denied, saying through his spokesman that he had recused himself from all decisions involving Boeing.

However, Air Force Secretary Heather Wilson’s own acknowledgement that the service had not wanted to buy new F-15s continued to raise speculation that Shanahan had played a role.

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Lawmakers stand ready to protect F-35 from F-15X budget threats Air Force leaders say their commitment to the F-35 is ironclad. Supporters of the program aim to ensure it stays that way.

The defense official speaking to reporters on Friday denied that Shanahan had any knowledge of when Boeing or any of its platforms was being considered during budget deliberations, though Shanahan was aware that discussions were happening broadly about the optimum mix of fifth-generation jets — like the F-35 — and fourth-gen platforms, which can include Boeing’s F-15 as well as Lockheed Martin’s F-16.

“CAPE ran the program budget review” that assessed whether to buy new fourth-gen jets, the official said.

“Working with the standard of conducts office, we put in place a pretty strict regime of keeping anything related to Boeing out of his purview during the program budget review process,” he added. “He was involved in broad capability discussions or broad force shaping discussions, [but] when it came to any specific platform that involved Boeing, those conversations were held strictly away from him.”

So why did CAPE push so strongly for buying additional fourth-generation jets?

The official pointed to two major factors. First was the need for additional capacity.

The average age of the F-15C/D fleet is 35 years, with some aircraft nearing the end of their service lives. FY20 budget documents note “SERIOUS structures risks, wire chafing issues, and obsolete parts” and add that “readiness goals are unachievable due to continuous structural inspections, time-consuming repairs, and on-going modernization efforts.”

CAPE considered accelerating procurement of the F-35, which in FY20 is limited to 48 units. However, its cost analysis — which pegs the cost of each F-15X at about $90 million for the aircraft and spares — found that F-35 operations and maintenance costs outweigh that of fourth-gen planes like the F-15, the official said.

The second argument in favor of buying new fourth-generation planes is that the national defense strategy establishes the need for both stealthy tactical aircraft that can penetrate into a contested zone, as well as planes with large payloads that can launch ordnance from standoff distances, the official said.

Out of the Air Force’s inventory, the F-15 in particular has that as a selling point. Of all the service’s fighters, it can carry the largest payload.

The defense official didn’t go into detail about what alternatives it considered or why it chose the F-15 over the F-16. Instead, he spoke more broadly about the need for industrial diversity in the run-up to the development of a sixth-generation fighter.

“Maintaining a diverse industrial base is in the best interest of the Department of Defense, not just in the [tactical aircraft] portfolio but in basically any other portfolio as well. So the kind of more diversity we can get there, the more competition we have, the better prices we have,” he said.

However, the decision puts pressure on F-35 manufacturer Lockheed Martin to decrease production and sustainment costs.

Last month at the Air Force Association’s air warfare conference, OJ Sanchez, Lockheed’s vice president for sustainment innovation and operations, said the company was on track for reducing the cost of an F-35A conventional mode to $80 million per jet by 2020, as well as to meet a $25,000-cost per flight hour by 2025.

But Wilson said the company is not making progress quick enough.