The Joint Strike Fighter has been described as the largest single defense program in history, with a potential market for 5,000-8,000 aircraft worth over $200 billion, when all potential export orders are included.
In November 1996, Boeing and Lockheed Martin were awarded contracts to build two Concept Demonstrator Aircraft — one CTOL version and one STOVL version — each. The aircraft were not intended to be fighter prototypes, but rather to prove that the selected design concepts would work, hence the use of X designations.
Boeing was assigned X-32 while Lockheed Martin received the designation X-35.
The Boeing X-32 used a novel airframe shape combined with a direct-lift STOVL configuration. The Harrier-style direct lift concept required the lift nozzles to be on the center of gravity of the aircraft. To achieve this, the engine was located in the front portion of the fuselage, with the vectoring nozzles immediately behind it, and a long exhaust duct leading back to the afterburner and pitch-axis thrust vectoring nozzle at the rear. The engine position and overall dimension limitations dictated a very short nose.
For the two CDA aircraft, the designation X-32A was allocated to the CTOL version and X-32B to the STOVL version. Unlike the Lockheed Martin X-35, there were no airframe changes required to demonstrate U.S. Navy aircraft carrier (CV/CTOL) approach capabilities — the X-32A performed both roles.
The X-32A featured a non-moving intake and wide-span wings with accentuated tip extensions. The X-32B featured a moving intake cowl that translated forward during hover to allow more air into the engine. The fuselage was slightly shorter and the wing span was narrower to reduce weight.
After the X-32 design was frozen, the planned Model 375 production version continued to evolve, gaining a conventional horizontal stabilizer and a stubby swept wing rather than the original delta wing. The engine intake cowl was also raked backward rather than forward.
Lockheed Martin X-35
The X-35 was the Lockheed Martin Joint Strike Fighter demonstrator, competing with the Boeing X-32.
The initial X-35A reflected the basic Air Force CTOL design, and was used for early flights before being modified into the STOVL version, designated X-35B. While Boeing proposed a direct lift STOVL design based on that used in the Harrier, Lockheed opted for a different approach in meeting the vertical flight requirements. Inspired by the Russian Yak-141, the X-35B incorporated a separate lift-fan that was shaft-driven by the F119 engine, allowing cooler exhaust temperatures during hover.
While the Boeing design was more conventional, Lockheed argued that their strategy was better in the long term since it offered more room for growth as the aircraft evolves. The second airframe was the X-35C STOVL demonstrator for the Navy. This model featured an enlarged wing of greater span and area for larger fuel capacity, as well as enlarged horizontal tails and flaperons for greater control effectiveness during low-speed carrier approaches.
The X-35 was selected as the winner of the JSF competition on Oct. 26, 2001.
The production aircraft is designated the F-35, and was later given the named F-35 Lightning II.
There are a number of differences between the X-35 and F-35, which was designed to be an operational weapon system. The forward fuselage was lengthened by 5 inches to make room for mission avionics, while the horizontal stabilizers were correspondingly moved 2 inches aft to retain balance and control. The diverterless supersonic inlet cowl shape changed from a four-sided to a three-sided shape and was moved 30 inches aft. To accommodate weapons bays, the fuselage section was fuller with the top surface raised by 1 inch along the centerline. Following the designation of the X-35 prototypes, the three variants were designated F-35A (CTOL), F-35B (STOVL), and F-35C (CV).
There are eight international program partners — the United States, United Kingdom, Italy, Netherlands, Australia, Norway, Denmark and Canada. Six Foreign Military Sales customers are also procuring and operating the F-35 — Israel, Japan, South Korea, Poland, Belgium and Singapore. On June 30, 2021, Switzerland announced it would purchase the F-35A; and while it was announced in 2020 that the United States would sell the United Arab Emirates the F-35 under the Foreign Military Sales program, there is Congressional action that could limit or cancel that agreement. Turkey was once part of the F-35 program, but was expelled in July 2019, after the Turkish government bought the Russian S-400 air defense system.