NEW YORK—Donald Trump skipped breakfast one recent morning, blow-dried his hair in his Trump Tower penthouse and headed out to his personal Boeing 757 jet, dubbed “Trump Force One” since he emerged as the Republican presidential front-runner.

Seated in his plane’s living room, with its pearlwood and 24-karat gold trim, Mr. Trump worked alone, watching the big-screen TV and reading the day’s political news, mostly about him, with no binders of policy positions or talking points in sight. Upon landing, he waded alone into a...