WASHINGTON — In keeping with his penchant for shocking the defense industry, President Donald Trump today seemingly revealed plans to develop a twin-engine version of the F-35 Joint Strike Fighter known as the F-55, as well as an upgraded “F-22 Super.”But former Air Force Secretary Frank Kendall said he was “mystified by Trump’s comments,” and analysts said that what the president described would be a highly challenging engineering endeavor on par with creating a clean-sheet aircraft. They also questioned the wisdom of spending scarce budget dollars on a twin-engine advanced F-35 when the newly announced F-47 is already planned to...