Some of the U.S. military's F-35 Joint Strike Fighters, which are the oldest of their kind, could become unflyable as soon as 2026 — after only 2,100 flight hours.

According to Popular Mechanics, the blame can possibly be placed on a design and production process that entailed starting to build the planes prior to the final design specifications being set.

A recent static (non-flying) version of the F-35B, which is used by the Marine Corps, developed structural cracks during durability testing, Popular Mechanics reports. The static test plane was reportedly built to the same standards as the early F-35Bs.

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Although each F-35B should have a service lifetime of 8,000 hours, this operational test on the F-35 seems to suggest that the earliest model jets could be limited to just 2,100 hours, according to Popular Mechanics.

The technical magazine blames the Pentagon's use of "concurrency" to field the F-35 for many of its woes. That concept means the Pentagon and Lockheed Martin began building the jets before the development process was totally finished. This was reportedly done for reasons of speed, but it comes with risks — including the potential for costly upgrades to the plane's software and hardware.

A spokesperson for Lockheed Martin gave Popular Mechanics the following statement:

"The F-35B has completed full-scale durability testing to 16,000 hours. Planned modifications and fleet management of the early contract F-35B aircraft will ensure that they meet the 8,000-hour service life requirement, and aircraft delivering today incorporate these design changes in the build process to ensure they’ll meet 8,000 hours or more."

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In October, the military grounded its entire fleet of F-35 stealth fighters after one of the jets crashed during a training mission in South Carolina.