Robert A. Lewis, the co-pilot of the Enola Gay, the B-29 that dropped the atomic bomb on Hiroshima, died of a heart attack Saturday in Riverside Hospital in Newport News, Va. He was 65 years old and had lived in Smithfield, Va., for two years.

Mr. Lewis, a native of Brooklyn, volunteered to join the Army shortly after the attack on Pearl Harbor. He was one of 12 servicemen with backgrounds in electronics who were chosen by the Army Air Corps to test the weapon systems on the B-29 Superfortress, then under development.

He was then transferred to the West Coast to fly the B-29 on simulated attack missions. When Col. P.W. Tibbets Jr. was chosen to command the B-29 to be used on the Hiroshima mission, he selected Captain Lewis, who had survived two crashes and had a reputation for remaining calm in moments of stress, as his co-pilot. Makeshift Diary

''If I live a hundred years, I'll never quite get these few minutes out of my mind,'' Captain Lewis said in his log of the Enola Gay's mission, written in pen and pencil on the back of War Department forms, on Aug. 6, 1945.

''Everyone on the ship is actually dumbstruck even though we had expected something fierce. It was the actual sight that we saw that caused the crew to feel that they were part of Buck Rodgers' 25th century warriors.''

Captain Lewis's log was bought at auction in 1971 for $37,000 by David Kirschenbaum, a dealer in rare books and manuscripts. It was acquired in 1978 by Malcolm Forbes, publisher of Forbes magazine, for $85,000, at that time a record price for an American autographed document.

After World War II, Mr. Lewis became a pilot for American Overseas Airways, flying across the North Atlantic until 1947. He then joined the Henry Heide Company in New York, a candy manufacturer that later moved to New Brunswick, N.J. He received a number of patents for improving candy-manufacturing machinery. In 1971 he joined the Estee Candy Company, and in 1981 he retired.

Mr. Lewis is survived by his wife, Mary Eileen; his daughter, Susan; and four sons, Robert Jr., John, James and Steven.

Robert Lewis