The F-35 Joint Strike Fighter (JSF), also called the Lightning II, is a strike fighter airplane being procured in different versions for the Air Force, Marine Corps, and Navy. The F-35 program is DOD's largest weapon procurement program in terms of total estimated acquisition cost. Current Department of Defense (DOD) plans call for acquiring a total of 2,456 F-35s for the Air Force, Marine Corps, and Navy at an estimated total acquisition cost, as of December, 2017, of about $325.1 billion in constant (i.e., inflation-adjusted) FY2012 dollars. U.S. allies are expected to purchase hundreds of additional F-35s, and eight foreign nations are cost-sharing partners in the program. The Administration's proposed FY2019 defense budget requested about $10.7 billion in procurement funding for the F-35. This would fund the procurement of 48 F-35As for the Air Force, 20 F-35Bs for the Marine Corps, 9 F-35Cs for the Navy, and continuing development. The proposed budget also requested about $1.3 billion for F-35 research and development. The Joint Strike Fighter was conceived as a relatively affordable fifth-generation aircraft that could be procured in highly common versions for the Air Force and the Navy. Initially, the Marine Corps was developing its own aircraft to replace the AV-8B Harrier, but in 1994, Congress mandated that the Marine effort be merged with the Air Force/Navy program in order to avoid the higher costs of developing, procuring, and operating and supporting three separate tactical aircraft designs to meet the services' similar, but not identical, operational needs. All three versions of the F-35 will be single-seat aircraft with the ability to go supersonic for short periods and advanced stealth characteristics. The three versions will vary in their combat ranges and payloads (see the Appendix ). All three are to carry their primary weapons internally to maintain a stealthy radar signature. Additional weapons can be carried externally on missions requiring less stealth. From a common airframe and powerplant core, the F-35 is being procured in three distinct versions tailored to the varied needs of the military services. Differences among the aircraft include the manner of takeoff and landing, fuel capacity, and carrier suitability, among others. The Air Force plans to procure 1,763 F-35As, a conventional takeoff and landing (CTOL) version of the aircraft. F-35As are to replace Air Force F-16 fighters and A-10 attack aircraft, and possibly F-15 fighters. The F-35A is intended to be a more affordable complement to the Air Force's F-22 Raptor air superiority fighter. The F-35A is not as stealthy nor as capable in air-to-air combat as the F-22, but it is designed to be more capable in air-to-ground combat than the F-22, and stealthier than the F-16. If the F-15/F-16 combination represented the Air Force's earlier-generation "high-low" mix of air superiority fighters and more-affordable dual-role aircraft, the F-22/F-35A combination might be viewed as the Air Force's intended future high-low mix. The Air Force states that "The F-22A and F-35 each possess unique, complementary, and essential capabilities that together provide the synergistic effects required to maintain that margin of superiority across the spectrum of conflict…. Legacy 4 th generation aircraft simply cannot survive to operate and achieve the effects necessary to win in an integrated, anti-access environment." The Marine Corps plans to procure 353 F-35Bs, a short takeoff and vertical landing (STOVL) version of the aircraft. F-35Bs are to replace Marine Corps AV-8B Harrier vertical/short takeoff and landing attack aircraft and Marine Corps F/A-18A/B/C/D strike fighters, which are CTOL aircraft. The Marine Corps decided to not procure the newer F/A-18E/F strike fighter and instead wait for the F-35B in part because the F/A-18E/F is a CTOL aircraft, and the Marine Corps prefers aircraft capable of vertical operations. The Department of the Navy states that "The Marine Corps intends to leverage the F-35B's sophisticated sensor suite and very low observable, fifth generation strike fighter capabilities, particularly in the area of data collection, to support the Marine Air Ground Task Force well beyond the abilities of today's strike and EW [electronic warfare] assets." The Navy plans to procure 273 F-35Cs, a carrier-suitable CTOL version of the aircraft, and the Marines will also procure 67 F-35Cs. The F-35C is also known as the "CV" version of the F-35; CV is the naval designation for aircraft carrier. The Navy plans in the future to operate carrier air wings featuring a combination of F/A-18E/Fs (which the Navy has been procuring since FY1997) and F-35Cs. The F/A-18E/F is generally considered a fourth-generation strike fighter. The F-35C is to be the Navy's first aircraft designed for stealth, a contrast with the Air Force, which has operated stealthy bombers and fighters for decades. The F/A-18E/F, which is less expensive to procure than the F-35C, incorporates a few stealth features, but the F-35C is stealthier. The Department of the Navy states that "the commonality designed into the joint F-35 program will minimize acquisition and operating costs of Navy and Marine Corps tactical aircraft, and allow enhanced interoperability with our sister Service, the United States Air Force, and the eight partner nations participating in the development of this aircraft." The F-35 is powered by the Pratt & Whitney F135 engine, which was derived from the F-22's F119 engine. The F135 is produced in Pratt & Whitney's facilities in East Hartford and Middletown, CT. Rolls-Royce builds the vertical lift system for the F-35B as a subcontractor to Pratt & Whitney. Consistent with congressional direction for the FY1996 defense budget, DOD established a program to develop an alternate engine for the F-35. The alternate engine, the F136, was developed by a team consisting of GE Transportation—Aircraft Engines of Cincinnati, OH, and Rolls-Royce PLC of Bristol, England, and Indianapolis, IN. The F136 was a derivative of the F120 engine originally developed to compete with the F119 engine for the F-22 program. DOD included the F-35 alternate engine program in its proposed budgets through FY2006, although Congress in certain years increased funding for the program above the requested amount and/or included bill and report language supporting the program. The George W. Bush Administration proposed terminating the alternate engine program in FY2007, FY2008, and FY2009. The Obama Administration did likewise in FY2010. Congress rejected these proposals and provided funding, bill language, and report language to continue the program. The General Electric/Rolls Royce Fighter Engine Team ended their effort to provide an alternate engine on December 2, 2011. Fuller details of the alternate engine program and issues for Congress arising from it are detailed in CRS Report R41131, F-35 Alternate Engine Program: Background and Issues for Congress . The F-35 is currently in low-rate initial production, with 280 aircraft delivered as of April 2018. At least 250 of those were in U.S. service. Four to five aircraft are currently delivered each month, with the production rate scheduled to increase to 120 per year by 2019. In keeping with the acquisition plan that overlapped development and production (known as "concurrency"), the F-35 was also in system development and demonstration (SDD), with testing and software development ongoing, from October 2001 until April 11, 2018. The SDD phase will formally continue until the end of Initial Operational Test and Evaluation, when a "Milestone C" full-rate production decision will be made. Significant developments since the previous major edition of this report (July 18, 2016) include the following, many of which are discussed in greater detail later in the report: In the latest Selected Acquisition Report, DOD disclosed an intention to acquire F-35s through multiyear contracting. From FY 2021 to the end of the program, the USAF production profile assumes one 3-year multi-year procurement (FY 2021-FY 2023) followed by successive 5-year multi-year procurements beginning in FY2024, with the required EOQ investments and associated savings. The Department of Navy (DoN) did not include EOQ funding in the PB 2019 submission for a multiyear in FY 2021-2023 for either the F-35B or F-35C. The DoN plans to reassess that decision in the coming FY 2020 budget cycle. Therefore, the DoN PB 2019 production profile assumes annual procurements from FY 2021-2023, followed by successive 5-year multi-year procurements from FY 2024 to the end of the program with necessary EOQ investments and associated savings. The F-35 Joint Program Office declared the 17-year System Development and Demonstration (SDD) effort complete on April 11, 2018. "(T)he developmental flight team has conducted more than 9,200 sorties, accumulated 17,000 flight hours and executed more than 65,000 test points." The end of the flight test effort does not mark the actual end of SDD, though; that will occur at Milestone C, following the completion of initial operational test and evaluation (IOT&E).