When and by who is pretty straightforward. Henry Ford created the modern automobile- that is to say, the automobile as a dominant means of transportation - and he also created the tractor. He had grown up on a farm, knew first-hand how much physical labor was involved in farming and wanted to make it easier. As with the automobile, he seems to have experimented with various designs and offered a few for sale before putting out the first mass-production model in 1917. Ironically, someone managed to anticipate his intentions and, in a sort of early cyber-squatting, created the Ford Tractor Company in 1915 that produced badly-designed and badly-made tractors. It not only resulted in Henry Ford not being able to offer tractors for sale under his own Ford name , but also the Nebraska Tractor Tests of 1919, in which the state forbid any tractors from being sold that did not pass quality tests. A common indicator of a good business idea is how many imitators it has: the tractor was enough of a good idea that within four years of its appearance there were enough different tractor manufacturers offering them for sale to make Nebraska want to pass laws about them. It was not limited to the US, either: Ford shipped them over to England, to help them with their labor shortage during WWI, and they quickly took over there, as well.

As to how much of an increase in production: there are a lot of different famers, different soils, and different crops. Tobacco farmers have long had a fondness for mules- they compact the soil much less than tractors, and those farmers use very deep, rich loam to grow a very high-value crop. Someone with a farm in western North Carolina also might have used a mule, at least into the 1950's, if they only had a small plot in hilly country ( or they might have gotten a walking tractor like a Gravely, invented in hilly West Virginia) . But on the big farms of the great plains, the tractor was immediately adopted, and there the rule of thumb seems to have been at least a factor of 3: a tractor could till three times the acreage that a mule could, in the same amount of time. Now, of course, it's far beyond that.

Vincent Curcio's Henry Ford is a good short introduction to the man...he was also responsible for the introduction of soybeans.