When David McCullough ’55 was a junior at Yale, nearly half a century ago, he stayed in New Haven over spring break to write a paper. Cooped up in Sterling Memorial Library, surrounded by piles of books, he learned to love what he calls the “thrill” of research.

“My junior year at Yale, I wanted to be a writer or a painter,” he said. “I love painting and I still do it. I’m a Sunday painter.”

But McCullough — the winner of a Pulitzer Prize and most recently the author of the number-one best-seller “John Adams” — has found fame in the dusty and sometimes obscure corridors of historical research. Back at Yale last Friday to lecture, he has climbed from undergraduate English major to major international historian.

He won the Pulitzer, the crown of any historian’s career, for 1992’s “Harry Truman.” And after 39 weeks on the New York Times nonfiction best-seller list, “John Adams” is still number six.

Jock Reynolds, the director of the Yale University Art Gallery, said he invited McCullough back to Yale to research John Trumbull, a Harvard graduate who is known primarily for painting the “Declaration of Independence.”

Speaking to a nearly full house in the art gallery’s auditorium, Reynolds called McCullough one of the most distinguished Yale graduates in any field.

McCullough, famously modest, described research as the lodestone that drew him forward.

“Discovering what was here, what was part of the Yale treasure trove, was one of the great thrills of my researching career,” McCullough said.

Trumbull donated the original art gallery building and many of his own works to Yale in 1832. McCullough said the University owns 67 paintings and 144 original drawings by Trumbull.

Deciding whom to research, McCullough said, has always been a matter of instinct.

“Something reaches out and takes ahold of you by the lapel, and pulls you in,” he said.

And part of the thrill of research, he said, comes from being able to hold centuries-old documents in his hand. He added that old cotton-mush paper is much more durable than modern wood-pulp paper.

“I pity future historians who have to work with decaying [wood-pulp paper],” he said.

McCullough said Trumbull’s most epic paintings, like the “Declaration of Independence” — painted as old age diminished his skills — have unduly overshadowed his smaller works. Fine portraits, rather the sweeping canvases on display in the Capitol rotunda, are the gem of his career.

“The small paintings are the great paintings,” he said. “They have color, vitality and heroic detail.”

He added that researching Trumbull at Yale is appropriate for a reason that has little to do with paper and canvas — John Trumbull was buried with his wife in the basement crypt of the old Yale Art Gallery.