http://www.flightglobal.com/news/articl ... 60-388468/
The above links to a
Flightglobal report that the USN needs more range from its AIM9X missiles. The article cites the need for a medium range missile for its F35s that is less susceptible to developing counter-measures than AIM120, the missile you would normally expect to be used for BVR engagements. I would guess that this is also to provide an agile missile to offset some of the F35's aerodynamic weaknesses should it get tangled up in a close encounter with a more manoeuvrable fighter. There is no mention of how big the upgraded missile might be but I surmise they are talking smaller than AIM120 so as to maximise the number that could be carried internally by F35, so helping it retain its low observable profile and so reducing the risk of getting into a turning fight in the first place.
LF