It contains a GAU-22, a four-barrel Gatling-type rotary cannon that spews death at 3300 25mm rounds per minute. This cannon was made especially for the F-35 and, according to General Dynamics Armament and Technical Products, is much more accurate than GAU-12/U — its classic predecessor. The latter, which uses five barrels, can fire at 4200 rounds per minute.
But why does the F-35B carry its optional cannon in a pod on its belly, you ask? Because — unlike the regular F-35A — this short take off and vertical-landing variant doesn't have space inside its fuselage. Unfortunately, carrying this optional weapon reduces the stealth of this type of Lightning II, which is supposed to be one of its core strengths.
The image was taken during tests on June 14. [The Aviationist]