Sony co-founder Masaru Ibuka wanted to listen to opera on business trips. He just needed something lighter to lug around an airport and on a plane.

Sony’s TC-D5 model, created in 1978, allowed him to listen to music through headphones in stereo. But at approximately 3.7 pounds and the size of a textbook, it was too heavy for Ibuka’s liking, according to Sony’s website.

Before Ibuka’s next trip to the U.S., Sony engineers modified the company’s compact Pressman recorder by removing its recording function and adding stereo playback.

Ibuka showed it to fellow co-founder Akio Morita, who believed pairing the product with lightweight headphones would appeal to young music lovers even if they couldn’t record.

“You can take your music out on the town with you,” he told young Sony employees in February of 1979.

The product that became the Walkman was beginning to take shape. Because he was targeting young buyers, Morita wanted it on store shelves before summer break.…