ST. LOUIS — Boeing is preparing to build F-15 fighter planes for the U.S. Air Force at its St. Louis County plant even though the military branch hasn’t bought the jet in over a decade.

The Chicago-based company began ramping up its F-15 production line near St. Louis after the Air Force submitted a nearly $8 billion budget request last month that included eight F-15s next year and 72 in the following four years. The request came as a surprise to many since the U.S. military has moved toward stealth fighters, such as Lockheed Martin's F-35, in recent years.

Prat Kumar, Boeing International’s vice president, told the St. Louis Post-Dispatch that the company is investing before Congress approves the budget request so it can respond quickly should the Air Force seek rapid field deployment.

Engineers and manufacturing experts recently met at the St. Louis County facility to determine how to efficiently assemble the fighter jet with its modern defense, radar and operating systems.

The first F-15 was first developed in the early 1970s, and foreign orders from Singapore, South Korea and Saudi Arabia have kept the Missouri manufacturing line running in recent years.

"With all the improvements we've done to the F-15 over the years, there's more interest in the F-15," said Andy Stark, manager of F-15 assembly. "We'd rather get ahead of the need versus waiting for the need to happen. So we're doing these studies so that way when the need occurs we've already got the business case and we're ready to pull the trigger."

The line is equipped to build about one F-15 a month, but Boeing officials believe that minimal modifications can increase production to up to three of the jets each month.

Some lawmakers have already expressed concern that the request for F-15s could come at the expense of Boeing's competitor, Lockheed Martin. The Air Force cut its plans to buy F-35s in the recent budget request from 54 to 48 for the fiscal year 2021 through 2023.

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Five senators from states where the F-35 is produced, including Republican Sen. John Cornyn of Texas, sent President Donald Trump and Acting Defense Secretary Patrick Shanahan a letter before the Air Force detailed its budget request that warned against funding F-15 planes at the expense of F-35s.