The flight that put the Boeing Company on course for disaster lifted off a few hours after sunrise. It was good flying weather—temperatures in the mid-40s with a slight breeze out of the southeast—but oddly, no one knew where the 737 jetliner was headed. The crew had prepared three flight plans: one to Denver. One to Dallas. And one to Chicago. In the plane’s trailing vortices was greater Seattle, where the company’s famed engineering culture had taken root; where the bulk of its 40,000-plus engineers lived and worked; indeed, where the jet itself had been assembled. But it was May...