A key component of your answer to this question should be a definition of the term "middle class" and how this may have changed over time. In the U.S. today, commentators and politicians often speak of the "middle class" as if it means everybody apart from the very wealthy. For instance, we hear social critics saying things such as, "Our elites have failed to help the middle class," as if the "middle class" represents the whole country. Or, one wonders if are they saying that to them, the poor do not matter, or that (incredibly) they believe there is no poverty in the U.S.? Historically, the terminology about class has been used somewhat differently. In Britain perhaps more so than in America, a clear distinction was made between "middle class" and "working class," the latter being broadly defined as people who do manual work, either skilled or unskilled, in either agriculture or industry. The other important point about the "middle class" is that it is by nature supposed to be upwardly mobile. Its members strive to become wealthy, to become part of the ownership class, in a manner that has historically been denied to the working class.
Apart from the precise definition of terms, the other part of your answer should recognize that from its inception the U.S. was a country in which ordinary people had greater opportunities for upward mobility than in Europe. There was seemingly an unlimited amount of land that the settlers could lay claim to, and the fact that most of it was already occupied by the indigenous people of the continent did not stop the Americans from taking it. At a time when agriculture was still the method by which most people made their living, the ideal in America was to own the land one farmed--or, unfortunately and tragically, in the Southern U.S., to own the land and to have it worked by enslaved people. With the growth of industrialization, increasing numbers of people came to live in towns and cities and to work in industrial facilities, in factories. With unionization in the late nineteenth and early twentieth century, working-class people themselves became upwardly mobile. The two World Wars brought Americans in contact with other parts of the world, and especially after World War II, there were new opportunities for education for the returning veterans, through government sponsorship such as the G.I. Bill. This led to a huge increase in the size of the middle class in the twentieth century. Men and women whose parents had been working class were now able to receive university educations and to do skilled work in education, in offices, in the fields of medicine, law, and the new computer industry.
Hopefully the above discussion will give you some ideas with which to answer your question for class.