Story highlights Trials aboard a Navy warship in May did not prove the F-35B is ready for combat, a Pentagon report says

The F-35B Lightning II is designed to replace several existing Marine Corps aircraft

Marines say the May testing verified several F-35B capabilities

(CNN) After nearly 15 years of schedule delays and highly criticized cost increases, U.S. military officials and defense contractors confidently proclaimed the much maligned Joint Strike Fighter program back on track when the Marine Corps declared its version of the F-35 fighter jet combat-ready last month.

However, the fifth generation stealth fighter jet may still have a number of maintenance and reliability problems that "are likely to present significant near-term challenges for the Marine Corps," according to a complete copy of a recent memo from the Pentagon's Director of Operational Test and Evaluation (DOT&E), released in a report by the nonpartisan government watchdog group Project on Government Oversight (POGO).

While military officials promoted the F-35's performance during Marine Corps operational test trials aboard the amphibious assault ship USS Wasp in May as a success and proof that the $400 billion fighter is ready for real-world combat deployment, the POGO report tells a very different story.

Photos: F-35 testing Sailors and distinguished visitors watch an F-35B Lightning II aircraft conduct vertical takeoff and landing flight operations aboard the amphibious assault ship USS Wasp in May 2015. Click through the gallery to see more images from the testing. Hide Caption 1 of 7 Photos: F-35 testing Two F-35B Lightning II fighters complete vertical landings aboard the amphibious assault ship USS Wasp during the opening day of the first session of operational testing in May 2015. As the future of Marine Corps aviation, the F-35B is designed to eventually replace all aircraft from three legacy Marine Corps platforms; the AV-8B Harrier, the F/A-18 Hornet, and the EA-6B Prowler. Hide Caption 2 of 7 Photos: F-35 testing An F-35 B Lightning II fighter lands on the flight deck of the amphibious assault ship USS Wasp during short take-off, vertical landing operations in May. A new Pentagon report says the testing did not adequately reflect conditions the aircraft would face in real operations. Hide Caption 3 of 7 Photos: F-35 testing Marines and sailors aboard the amphibious assault ship USS Wasp secure and refuel an F-35B Lightning II fighter after its arrival for the first session of operational testing. Hide Caption 4 of 7 Photos: F-35 testing Vice Adm. William Hilrides, commander of Naval Sea Systems Command, holds up two fingers to indicate to the F-35B Lightning II pilot to power up for takeoff aboard the amphibious assault ship the amphibious assault ship USS Wasp in May. Hide Caption 5 of 7 Photos: F-35 testing An F-35B Lightning II takes off from the flight deck of the amphibious assault ship USS Wasp during test operations in May. Hide Caption 6 of 7 Photos: F-35 testing Sailors and Marines remove a generator for the F-35B Lightning II aircraft from an MV-22 Osprey assault support aircraft aboard the amphibious assault ship USS Wasp during operational testing in May. A new Pentagon report says the testing highlighted maintenance challenges for the F-35. Hide Caption 7 of 7

Not only did the six F-35Bs used in the demonstration, referred to by the Marines as Operational Test 1, fail to achieve the number of required flight hours necessary to be declared combat-ready, but, in fact, the DOT&E found the trials, "did not -- and could not —demonstrate that Block 2B F-35B is operationally effective or suitable for use in any type of limited combat operation, or that it was ready for real-world operational deployments, given the way the event was structured," the report says.

To qualify as a true operational test with results that would allow the Department of Defense to determine whether or not the F-35B is operationally effective and ready to be deployed, testing would have to be conducted under conditions more representative of real-world operations, the report says.

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