Jockey was the first company to come up with brief-style underwear for men, and the design dates from the 1930s. Guys were initially uneasy about the new, then considered very minimal, design, when the normal undewear for men was the so-called "Union suit", much larger, often with a trap door in the rear. As I understand the story, sales were slow initially especially in less urbanized areas , but men found the minimal jockey shorts to be very comfortable and by the 1940s they were widely worn See this link. Go to the 1930s
http://www.jockey.com/en-us/CorporateInfo/History/
Anyhow, what the Speedo company did was simply adapt the Jockey Brief design for use as swimwear. Obviously cotton was not going to cut it as swimwear, and thats when the newly developed nylon came into play. Others had experimented with brief-style swimwear prior to Speedo. In the 40s there were actually brief-style swimuits for men made out of WOOL of all things!
The rest is history, mainly the transition to Lycra. In the late 1980s competition swmmers thought the smaller the brief the faster they could swim, so this was the period of time you saw the really tiny briefs, particularly worn by competitive swimmers from Europe...France, Italy, 1/4"-1/2" inch siides. Then the early 90s thongs became popular with many men but that is a story for another day. I remember sorting hrough a rack of thongs for men at Macy's. I used to wear these regularly at Hotel/Motel hot tubs.I would put a swimming brief on over the thong and then discard the brief as I entered the hot tub. Great fun!
Sebbie