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Hill Air Force Base on Friday said it has completed modifications on two Marine Corps F-35 jets.

The Marine Corps F-35s differ from their Air Force and Navy counterparts in one big way — they are capable of vertical take off and landing.

Hill conducts maintenance on F-35s and the Marine Corps jets that arrived in February. The modifications performed by Hill have been said by the Pentagon to include software improvements.

Hill released photographs of the Marine Corps F-35s on Friday and said the aircraft completed a successful test flight a day earlier above the Utah Test and Training Range. A test pilot returned the aircraft to Hill and landed vertically.

Any step in F-35 production and readiness has become a milestone. The project is years behind schedule and is said to be the most expensive weapon in American history.

The 570th Aircraft Maintenance Squadron units performed the modifications. A squadron of 72 F-35s are expected to begin arriving at Hill this fall to reside with the base's 388th Fighter Wing.

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