The world could gain its first ever trillionaire this week, as Elon Musk takes his rocket and satellite company SpaceX public Friday in the biggest IPO in history. His wealth explosion has inspired his devotees, exhilarated his investors, vexed the skeptical and enraged those calling for higher taxes on the ultra-rich.

Yet the making of the world’s first trillionaire, if sudden—Musk was worth less than $25 billion just six years ago and $150 billion five years ago—has always been inevitable.

America has been marching toward this moment since John Jacob Astor arrived in the new nation in the 1780s and began trading in fur, tea and Manhattan real estate, likely becoming the first self-made millionaire in the process. American ingenuity soon swelled the millionaire class to include titans of tobacco, steel, banking, even refrigerated railcars.

It took more than a century to go from first millionaire to first billionaire. But by 1918, when a five-month-old Forbes magazine surveyed "the foremost bankers in the country,” John D. Rockefeller had achieved the feat, having amassed a fortune of $1.2 billion (roughly $29 billion in today’s dollars). “Mr. Rockefeller’s wealth,” Forbes founder B.C. Forbes wrote at the time, “if it could be turned into cash and distributed equally—which it couldn’t—would give every man, woman and child in the United States $10 [$240 today].” (If Musk’s money were similarly turned into cash and distributed, each American would get $2,300.)

Henry Ford soon followed Rockefeller, yet for much of the 20th century, as corporate America developed but taxes on the wealthy remained high, billionaires were few and far between. A handful of famous faces captured imaginations: the eccentric Howard Hughes dazzled the public with aviation feats and Hollywood romances. “He wanted to be the top golfer, the top aviator, he wanted to establish records,” said the former longtime head of Hughes’ business empire, in an interview with Forbes about his tell-all........

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