 FACT: In addition to the F35 jets only having 1 on board cannon, the  on board cannon is only available in the F35A variant of the jet.  This variant is the Conventional Take Off and Landing variant of the  F35, and would not be capable of deploying its turbofan and  hovering as it was seen to do in the film. The Short Take Off and  Vertical Landing variant of the F35 can be fitted with a cannon but  this would be mounted on one of the external wing hard points.