Boeing celebrated the successful maiden voyage of its 777X, the largest twin-engine passenger jet at Boeing Field outside Seattle on Saturday. The new jet, which is a record-breaking 252 feet long, is now the longest commercial jet. It will officially be called the 777-9 when it is introduced to commercial flight after more testing. The test flight took place against a backdrop of undelivered 737 MAX jets, which have been grounded after two fatal air disasters and controversy over the aircraft’s automated systems. “It’s a proud day for us,” the chief executive of Boeing’s commercial airplane unit, Stan Deal, said. “It made all of our employees proud one more time of who we are and what we get to do, by flying a brand new airplane that is going to change the world one more time.”