Is there a known draft of the United States constitution that's significantly different from the final version we know today?

There was the Articles of Confederation prior to the adoption of the U.S. Constitution, and there were different plans suggested to those in attendance at the Constitutional Convention where the U.S. Constitution was debated over the course of a few months and eventually presented to the states for adoption. In addition to this, there was the tacking on the Bill of Rights as requested by the states before they would officially adopt the U.S. Constitution.

The Articles of Confederation were adopted on March 1st, 1781. The Articles formed a government that closely resembled what the states had already been working with during the Revolution under the Continental Congress. Unfortunately, following the war, the Articles of Confederation ran into some issues. The Articles did not provide for a strong central government, and as such the federal government was struggling to keep states in line, deal with pressing issues such as Shays' Rebellion, and to collect taxes. Due to this, there were attempts made to bring the states together to amend the Articles of Confederation.

However, it became clear once the states had finally convened (with the exception of Rhode Island) that the Articles were far too weak to be made effective through simple changes. Instead, they opted to convene as a Constitutional Convention in which they would draft a new Constitution that would provide for a much stronger federal government.

During the convention, there were many debates and nearly every aspect of the Constitution that we know today was considered in length. One of the plans presented during the Constitutional Convention was the Virginia Plan. It was primarily written by James Madison and presented by Governor Randolph. This plan suggested the formation of a three-branch governmental system and specified a bicameral legislature. According to the Virginia Plan the lower house of Congress would based wealth and population. Those serving in the lower house would be elected by the people, while the upper house would be elected by the lower house.

This plan was particularly popular among larger states which would benefit from their larger populations by gaining more seats in Congress. This made the plan greatly unpopular among smaller states who worried that they would be constantly overpowered by the larger states in such a government.

The New Jersey Plan was also proposed, and suggested adding two branches of government to the already existing Articles of Confederation. It also proposed that the executive branch should be made up of a panel or group of people. What the New Jersey Plan did not change was the function of the current Congress under the Articles of Confederation, which gave each state one vote, rather than apportioning representation by population.

Ultimately the Virginia Plan won out and was later changed throughout the convention to resemble what has become the United States Constitution that we know of today, featuring three branches of government, a bicameral legislature (the lower house apportioned according to population, and the upper house apportioned equally to each state).

There were also a number of other issues that were addressed in depth during the Constitutional Convention, all of which could have changed the form that the United States government would take or the ways in which it functions.

The United States Constitution was eventually adopted by the states over time and states that were originally wary of the Constitution were able to get the Bill of Rights added which provided greater protection of states' and peoples' liberties.