It was 31 years ago this month that Sony changed the way people listen to music when it began selling the Walkman on July 1, 1979.

Portable music devices were nothing new. Lightweight transistor radios had been available since the 1950s, but the sound quality was poor. There were 8-tracks and cassette players that provided good sound quality, but the bulky devices were “portable” more in name than practice.

Such was the state of portable music in the late 1970s when Sony’s co-founder, Masaru Ibuka, began wondering if a better device could be built. Ibuka often carried a Sony TC-D5 stereo cassette recorder with him on international flights to listen to classical music. The TC-D5 was heavy, so Ibuka suggested the recording capabilities be removed to lighten the load.

Sony engineers rigged a playback-only device for Ibuka to use on his next trip. The headphones were bulky and the batteries ran out mid-way through the flight, but Ibuka liked the concept.

Within 4 months, engineers had perfected the device and Sony began selling the Walkman TPS-L2 in Japan, for 30,000 Yen, or about the equivalent of $150 in 1979 dollars.

Sony originally produced 30,000 TPS-L2 devices, but sold just 3,000 in the first month. By the end of August 1979, however, the company had sold out its inventory and would go on to sell 50 million Walkmans by 1989.