The F-35 Joint Strike Fighter has reportedly been racking up kills against older warplanes during U.S. military drills in Nevada — even the F-15, whose record in real combat is a flawless 104 to zero. Now the two jets are heading into a fierce dogfight, one that doesn’t involve missiles or guns.

The battle between Lockheed Martin’s F-35 and Boeing’s F-15EX is being fought by lobbyists in and around Congress, which is beginning to review the Pentagon’s fiscal 2020 budget request. Tens of billions of dollars are up for grabs over the coming decade.

This week, Pentagon officials proposed buying new F-15s for the first time since 2001, even though top Air Force officials have said as recently as two weeks ago that they didn’t necessarily want the the planes. For nearly two decades, Air Force officials have argued against buying so-called fourth-generation planes, preferring for stealthier fifth-generation planes with newer technology.

The proposed F-15 purchase is rather small: eight jets in 2020 and a total of 80 through 2024. By comparison, the Pentagon wants to buy 78 F-35s in 2020, with 48 going to the Air Force.

But Pentagon budget documents also signal that the Air Force could buy hundreds of F-15s over the next decade. A tranche of 144 planes would “initially refresh” squadrons that fly Cold War-era F-15C Eagles designed for air-to-air combat. And the plane has the “potential to refresh the remainder of the F-15C/D fleet and the F-15E fleet.” In all, that’s more than 400 planes.

That was enough to draw a full-court press from Lockheed. One day after that announcement, company officials began circulating a three-page white paper detailing the “F-35’s decisive edge” over unnamed fourth-generation warplanes. Defense One reviewed the white paper.

Lockheed’s arguments boil down to bang-for-the-buck: The F-35 will cost about the same or less than the F-15 soon (the long-criticized price has in fact been coming down), its operating costs will be less than the F-15’s within six years, and it can fly a more diverse set of missions.

Boeing’s argument: The F-35 was never intended to replace the air-to-air F-15C — but the F-15EX could do so while expanding those squadrons’ capabilities. Pilots would not need extensive training to fly the jet, which could carry heavy loads of weapons, plus Eagle bases would not need major infrastructure upgrades. And the new F-15EX is multirole, similar to the F-15E Strike Eagle, meaning that it could strike targets in the air, on the ground or at sea.

Boeing has been pitching new F-15s to the Air Force on and off for more than a decade, most recently offering a similar version of the plane it builds for Saudi Arabia and Qatar. The latest effort started to pick up steam last summer.

The idea was embraced within parts of the Air Force, but not by top Air Force leaders. Air Force Secretary Heather Wilson acknowledged on Feb. 28 that the planes were not in the service’s initial budget plans.

But analysis by the Joint Staff and Pentagon Cost Assessment and Program Evaluation Office “on the kinds of capabilities that we require in the aviation realm” led officials to recommend buying the F-15EX, a senior defense official said.

Elaine McCusker, the Pentagon deputy comptroller, said Tuesday that former Defense Secretary Jim Mattis made the decision to include the F-15EX in the Defense Department’s budget request.

“The F-35 remains a critical program for the joint force as we look to the future and the kinds of capabilities we require,” Lt. Gen. Anthony Ierardi, director of force structure, resources and assessment on the Joint Staff, said Tuesday. “The F-15EX provides additional capacity and readiness, especially in the near years to mid years, as we look at the threats and the kinds of combat potential that we needed to bring to bear.”

Whether Congress agrees with that rationale is yet to be seen. In February, five Republican senators — all with ties to Lockheed F-35 manufacturing work or F-35 bases — sent a letter to President Trump in opposition of the F-15EX.

“We are extremely concerned that, over the last few years, the DoD has underfunded the F-35 Program and relied on Congress to fund increases in production, sustainment, and modernization,” wrote the group, led by Sen. John Cornyn, R-Texas. “In order to meet the overmatch and lethality goals laid out in the National Security Strategy, the DoD needs to make these investments in the F-35 to affordably deliver and operate this fifth-generation fighter fleet. The F-35 is the most affordable, lethal, and survivable air dominance fighter, and now is the time to double down on the program.”

The 2020 budget request includes $11.2 billion to buy 78 F-35s — 48, which would be Air Force jets. That money would also go toward improving jets already built. Lawmakers have routinely added F-35s to the Pentagon’s request. For instance, last year they added 16 planes to the 77 requested by the Defense Department.

The 2020 budget request includes $1.1 billion for the eight F-15EX jets. Some of that money would go toward standing up the production line.

About a month after Bloomberg first reported in December that eight F-15EX jets would be in the budget request, Marillyn Hewson, Lockheed’s CEO, said Pentagon leaders told her that F-15 buys would not be at the expense of the F-35.

“The combat proven F-35 is the National Defense Strategy in action and the program continues to see strong support throughout the Pentagon, the U.S. Services, Congress and the White House,” the company said in an emailed statement.

Pentagon officials have been insisted that any F-15EX buys would not eat into planned F-35 buys. In all, the Pentagon plans to buy a total 2,443 jets over the coming decades.

“If Congress changes that to all F-35s, they’ll be all F-35s, we understand that,” Maj. Gen. David Krumm, director of strategic plans in the office of the deputy chief of staff for strategic plans and requirements at the Pentagon, said Thursday at the Mitchell Institute. “But based upon the resources we have and the ownership costs of the platforms, we think that this is the best way that we can present the nation’s Air Force and the best way we can get to a capabilities and capacities that we have.

“If we have more resources, I think we need to have a conversation about what it is we go for,” he said. “But based upon the resources that we have, we think that this is the right way to go."