Rain created the first ODI: a washed-out Ashes Test at the MCG, 5 January 1971, and 40 overs a side.
5 January 1971, the MCG. The third Ashes Test was washed out without a ball bowled, so officials, Don Bradman among them, staged one innings each, 40 overs a side, eight balls an over. 46,000 came. England made 190; Australia chased it with five wickets and 42 balls to spare. John Edrich made 82: the first Man of the Match in ODI history.
The next official ODI took more than 19 months: Old Trafford, August 1972, 55 overs a side. Dennis Amiss made 103 on debut, the format's first century.
Borrowed from English county cricket: the Gillette Cup (1963) and the Sunday League (1969). Three years later, the men's game had a World Cup.