“They were 16 that summer, and schoolmates — three American boys in a borrowed Packard. They had a rifle, a tent, and a camp stove. . . . They left Indy at dawn on July 31, 1939, planning to be gone a month. The record tells us that all three cheered as they turned onto Washington Street and the windshield compass spun to ‘W.’ They were headed for the land of cowboys and cattle rustlers, Indians and screaming wildcats.” — Ginger Strand in “Vonnegut on the Road” essay, “Tin House” magazine

A couple of years after Bud Gillespie took a memorable adventure out West with his good friend Kurt Vonnegut, the outbreak of World War II turned their lives upside-down.

But during that golden summer of 1939, Mr. Gillespie, Vonnegut — who went on to become a giant in American literature — and their buddy George Jeffrey left their hometown of Indianapolis for a road trip on which they called themselves “The Rover Boys.”

A resident of Downers Grove since 1966, Mr. Gillespie — the last surviving Rover Boy — died Aug. 1 at 96 at Beacon Hill senior community in Lombard, said his daughter Mary Jane Haley.

The journey “was pretty formative for Vonnegut,” said Ginger Strand, author of the 2015 book “The Brothers Vonnegut.” “And road trips do come up in Vonnegut. A road trip through the country becomes a kind of journey through the national mythology. It definitely happens in ‘Breakfast of Champions.’ ”

Kurt Vonnegut (center, in cowboy hat) with Bud Gillespie (left) and George Jeffrey (right), as they headed on a road trip out West at 16. | Photo courtesy of Hawthorne Publishing of Indiana and Majie Alford Failey’s book “We Never Danced Cheek to Cheek, the Young Kurt Vonnegut in Indianapolis and Beyond”

After returning from their adventure, Mr. Gillespie entered the Army, landing on Omaha Beach on D-Day, according to his family. Jeffrey became a WWII pilot. And Army Private First Class Vonnegut was captured and survived the bombing of Dresden by taking shelter with other POWs in an underground slaughterhouse — inspiration for his novel “Slaughterhouse-Five.”

“It was a lifelong friendship,” said Mr. Gillespie’s daughter. “They were all fellow Indy boys, Hoosier guys.”

“Kurt was very connected to his Indianapolis friends his entire life,” said Strand.

Bryant “Bud” Gillespie was born in Indianapolis, where he attended Shortridge High School with Vonnegut, who died in 2007; and Jeffrey, who died two years ago.

Strand studied unpublished letters at Indiana University that Vonnegut wrote home about the adventures of the Rover Boys. They say young Bud was entranced by horseback riding and finding the right 10-gallon hat.

“Bud is obsessed with being a cowboy, and he’s out riding every day,” Strand told the Sun-Times.

She wrote about Vonnegut’s “Rover Boys” letters in 2015 for the literary journal “Tin House” and in 2016 for “This Land Press.”

In “Tin House,” Strand wrote: “The highway was lined with billboards advertising roadside wonders: historic markers, Indian mounds, novelty statues, sacred miracle caves. Bud was mesmerized by all of them. He insisted on stopping at Mark Twain’s cave near Hannibal, Missouri, but George and Kurt dragged him away. They were eager to get to the real West and start their thrilling adventures.”

In 2007, Mr. Gillespie described the odyssey to Charles J. Shields, author of the 2011 Vonnegut biography “And So It Goes.” They left Indianapolis with a tent, a few rifles and not much else. The youths “went through Denver, went up Pikes Peak, Mesa Verde, canyon at Gunnison, and New Mexico,” Mr. Gillespie said. “Saw the Indian dances in Gallup, New Mexico. . . went into Mexico. Went to Carlsbad Caverns.”

Asked how their money held out, Mr. Gillespie replied, “Well, if you can stand eating sardines and crackers and Campbell’s soup, then it was all right.”

At one point, Bud used his driving skills to escape a carload of drunken laborers who chased the trio near the Colorado River, Strand said. “Fortunately for literature,” she wrote, “the workers’ Ford proved no match for the Packard.”

A character in the Vonnegut short story “The Manned Missiles” is named Bryant, “Bud” for short. Shields said, “Kurt liked to include the names of friends in his fiction.”

Mr. Gillespie, who got his bachelor’s degree at Washington & Lee University, became a vice president of Nightingale-Conant Corp., a motivational company in Wheeling for which his daughter said he recorded radio messages titled “A Time to Remember” and “A Thought for Today.” He retired in 1989.

Mr. Gillespie is also survived by his daughter Carol Dlesk, sons Thomas and John, 10 grandchildren and five great-grandchildren. An October memorial service is planned.

Strand wrote that, as the Rover Boys headed home, “Their car radio informed them that Hitler had invaded Poland. World War II had begun. The devil was at loose in the world, a devil that would end up trapping Vonnegut in….the Dresden basement called Slaughterhouse Five.”