America's most expensive weapons system everjust hit another snag.The F-35 Lightning II, Lockheed Martin'sfifth-generation fighter jet, is expected tomiss another crucial deadline on its march to combat readiness.On Tuesday, the Pentagon acknowledged that thejet would stumble pass its operational testing phase."The target was the middle of 2017, but it's clear we're not going to make that,"said Frank Kendall, the Pentagon's top acquisitionofficer said.The new scheduledate,according to Kendall, islikely to occur in 2018.Themid-2017 target was already a postponement from the original target date because ofsetbacks with the F-35'ssixth and final software release, referred toas Block 3F.The Block 3F softwareis part ofthe 8 million lines ofsophisticated software code that underpin the F-35.In short, if the code fails, the F-35 fails.The latest setback for the F-35 stems froma 48-pageDecember 11 reportfrom Michael Gilmore, thePentagon'stop weapons tester.According to Gilmore,the stealth fighter won'tbeready by its July 2017 deadline.As first reported by Aviation Week, the DoD report says"the rate of deficiency correction has not kept pace with the discovery rate," meaning more problems than solutionsarearising fromthe F-35 program."Examples of well-known significant problems include the immaturity of theAutonomic Logistics Information System(aka theIT backbone of the F-35), Block 3F avionics instability, and several reliability and maintainability problems with the aircraft and engine."One recommendation Gilmore gives for the F-35's latest woesis to triple the frequency of weapons-delivery-accuracy tests, which are executed once a month.Adding more teststo the troubled warplanewill most likely add to the cost overruns and schedule delays, butGilmore saysdecreasingtesting to meet deadlines will put "readiness for operational testing and employment in combat at significant risk."According to the DoD report, the Block 3F software testing beganin March, 11 months later than the planned date.The nearly$400 billionweapons program wasdeveloped in 2001 toreplace the US military's F-15, F-16,and F-18 aircraft.Lockheed Martin's "jack-of-all-trades"F-35swere developed todogfight, provide close air support, executelong-range bombing attacks,andtake off from and land on aircraft carriers ' allthe while usingthemost advanced stealth capabilities available.Adding to the complexity, Lockheed Martin agreed to design and manufacture three variantF-35s for different sister service branches.The Air Force has the agile F-35A; the F-35B can take off and land without a runway, ideal for the amphibious Marine Corps; and the F-35C is meant to serve onthe Navy's aircraft carriers.Despite the Block 3F software setback, the Marine Corps last yeardeclared an initial squadron of F-35s ready for combat, making itthe first service branch to do so.The standard for readinessthe Marines used, referred to asinitial operational capability,isdetermined separately by each service branch when the aircraft has successfully demonstrated various capabilities.IOCs are announced prematurely, however, in that all tests and upgrades to the aircraft, such asthe Block 3F software update, have not necessarily been completed.Still, Gen. Joseph Dunford, then the commandant of the Marine Corps,in Julydeclaredinitial operational capability for10 F-35B fighter jets.TheAir Force is expected todeclare IOC for its F-35As later this year, and the Navy plans to announce IOC for the F-35Cs in 2018.Even so, America's most expensive warplane's turbulent march to combat readiness isfar from over.Here's the full report from the Department of Defense:SEE ALSO:15 of the most expensive projects abandoned by the US militaryJoin the conversation about this storyNOW WATCH: This is how pilots train to fly America's most expensive fighter jets Click here to read full news..