According to Wiki:
The first jungle gym was invented in 1920 and patented by lawyer Sebastian Hinton in Chicago.It was sold under the trademarked name Junglegym. The term "monkey bars", was first documented in 1955, though Hinton's initial patent of 1920 appeals to the "monkey instinct" in claiming the benefits of climbing as exercise and play for children, and his improvement patents later that year refer to monkeys shaking the bars of a cage, children swinging on a "monkey runway", and the game of "monkey tag". Hinton's father, a mathematician, had built a similar structure from bamboo when Hinton was a child; his father's goal was to enable children to achieve an intuitive understanding of 3-dimensional space through a game in which numbers for the x, y, and z axes were called out and each child tried to be the first to grasp the indicated junction. Thus the abstraction of Cartesian coordinates could be grasped as a name of a tangible point in space.
Hinton's second prototype jungle gym is still standing at Crow Island School.