In potential reductions to major weapons systems, Mr. Panetta said he was considering cutting the purchases of the F-35 Joint Strike Fighter, a radar-evading jet for the Air Force, Navy and Marines that is projected to cost nearly $400 billion for more than 2,400 planes over the next two decades. He suggested he might slow or cut back production, although the final decision may be to protect that jet program and identify cuts in other weapons purchases.