(Bloomberg) -- The Pentagon will request 61 F-35s in its next budget, 33 fewer of the stealth jets from Lockheed Martin Corp. than previously planned, according to people familiar with the spending blueprint.

The U.S. Defense Department had planned to fund 94 of the fighters in fiscal 2023, up from the 85 in this year’s budget, according to the most recent “Selected Acquisition Report” on its costliest program.

The proposed reduction for the F-35 may be the most controversial procurement item in a national security budget request that’s expected to top $770 billion for the year that begins Oct. 1. 

The F-35 is currently being deployed to Eastern Europe in response to Russia’s invasion of Ukraine. Six F-35s from the Air Forces 34th Fighter Squadron are flying “air policing” missions from Estonia and Romania. German Defense Minister Christine Lambrecht announced Monday that Germany would buy 35 of the warplanes.

But the proposed slowdown in purchases may raise questions among lawmakers, Lockheed investors and overseas customers about a lessening of the U.S. commitment to a program projected to cost $398 billion in development and acquisition plus an additional $1.2 trillion to operate and maintain the fleet over 66 years. The people familiar with the budget plan asked not to be identified in advance of the budget release in coming weeks. 

The rationale for the reduction won’t be officially explained until the proposed Pentagon budget is made public. But the request comes as negotiations with Bethesda, Maryland-based Lockheed over the next F-35 contract -- for about 400 planes -- are going slower than anticipated. And F-35s remain hobbled by flawed execution of a crucial upgrade of their software and hardware capabilities that’s estimated to cost $14 billion.

Boeing’s F-15EX

Along with the proposed F-35 reduction, the Air Force will request 24 non-stealthy F-15EX jets built by Boeing Co., up from 14 planned in the fiscal 2021 budget. The EX model carries more ordnance than the F-35 and is estimated to be cheaper to fly. Still, the service plans to purchase many more F-35s than EX jets.

Asked about the cutback in planned F-35 purchases, Laura Seal, spokeswoman for the Defense Department’s F-35 program office, said the budget request can be discussed “once it is delivered and released, but not before.” Spokespersons for the Air Force, the largest customer for F-35s, and the Navy made similar comments.

The Air Force will request 33 F-35As instead of the 48 planned, the Navy will seek 13 of its version instead of 26 and the Marine Corps will request 15 rather than 20.

The F-35’s “Block 4” software and hardware upgrade is currently being installed on deployed jets even though it’s “immature, deficient and insufficiently tested,” according to an assessment by the Pentagon testing office released in January. Aircraft operators “identified deficiencies in weapons, fusion, communications and navigation, cybersecurity and targeting processes that required software modification and additional time and resources, which caused delays,” it said.

The aircraft flying missions now, including those in Europe, are described as capable by Pentagon officials, however. “We understand the threats that the F-35 is going up against today,” Lieutenant General Eric Fick, who heads the F-35 program office, told reporters this month. “We understand the threats largely propagated throughout Europe, and those were the threats that the airplane was developed to counter.”

A person familiar with the Air Force’s rationale for purchasing fewer planes said it shouldn’t be seen as stepping back from the service’s long-stated goal to buy 1,763 F-35s. Instead, the person said, it’s a matter of slowing purchases until the full Block 4 capability can be delivered on new jets in order to minimize the cost to retrofit them.

In addition, the F-35 is at least 14 months from starting and completing a much-delayed combat simulation to test its performance against the most advanced Russian and Chinese air defense and aircraft threats it faces now and are likely in the future. The simulated sorties were to take place in 2017, then 2020 and are now planned for sometime between June and September of 2023, Fick told reporters this month. 

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