F-35B Lightning II
The F-35B Lightning II version of the F-35 Joint Strike Fighter has, as its most important difference from the rest of the F-35 family, short takeoff and vertical landing (STOVL) capability. This is a requirement to replace the various Harrier versions being replaced in the the Italian Navy, the U.K. Royal Air Force and Royal Navy, and the United States Marine Corps.
It is the world's first operational supersonic STOVL aircraft, yet will have significant stealth characteristics. Rather than an internal gun, it will have a stealthy, belly-mounted 25 mm missionized gun pod and a combat radius of more than 450 NM- nearly two times that of legacy STOVL strike fighters.
STOVL control and propulsion
To have a lift greater than the total weight, the F-35B combines as shaft-driven lift fan with a vectored main engine exhaust nozzle. This combination is essential to achieving the desired performance; the fan provides an additional 14,000 pounds of lift beyond what thrust vectoring would provide by itself. 
Weight has been the bane of the "B" design, which took longer to achieve than that of the "A" and "C" models.
ComparisonsCarrying weapons internally, as required by stealth, and an increased fuel load for greater range make the F-35B larger and heavier than Harriers and the comparable Russian Yak-38. The followon Yak-141, with dual vectored engines, was not successful.
- "F-35B Joint Strike Fighter (JSF)", Globalsecurity.com