Just before 3 p.m. on Dec. 18, a surprising visitor showed up at 's military aircraft facility in San Antonio, Texas: First Buddy Elon Musk.

The billionaire tasked by US President Donald Trump to eliminate government waste and inefficiency had swooped in on his Gulfstream G650 to inspect the new version of Air Force One. The next iteration of the fabled presidential jet is years behind schedule, $2.7 billion and counting over budget and a major headache for Boeing Chief Executive Officer .

Musk is no ordinary Trump emissary. After all, his rocket company SpaceX is a fierce Boeing competitor. ...