WASHINGTON — The Pentagon on Friday grounded its fleet of F-35 jets after discovering a cracked engine blade in one plane.

The problem was discovered during what the Pentagon called a routine inspection at Edwards Air Force Base, Calif., of an F-35A, the Air Force version of the sleek new plane. The Navy and the Marine Corps are buying other versions of the F-35. All versions — a total of 51 planes — were grounded. None of the planes have been fielded for combat yet.

‘‘The F-35 is a huge problem because of its growing, already unaffordable cost and its gigantically disappointing performance,’’ said Winslow Wheeler, of Project on Government Oversight, a watchdog group.