A felony was originally a serious breach of the contract between vassal and lord in the days of feudalism; often they involved something to do with property and involved forfeiture of goods and property. A murder, under the original definition, was not a felony, although it was still a heinous at punishable by death.
As the Common Law developed it produced a meaning of felony more divorced from the feudal definition and was basically applied to any crime under the Common Law that was punishable by death, forfeiture of goods and property, and/or corruption of blood, while the feudal meaning, while not immediately disappearing, faded over time, except when it came to treason and petit treason (disloyalty to husband, master, etc.).
Over time felonies became more associated with the death penalty than with forfeiture, but the latter definition still remained. The traditional Common Law felonies were murder (degree did not matter and I don't think it was used then; murder was murder, as it should be), manslaughter, arson, robbery, rape, sodomy, mayhem, and larceny; treason and petit treason remained felonies as well. Felony also came to mean crimes designated such and worthy of death or forfeiture (usually the former) by statute.
When we were founded, this was the status of the term, although in America, forfeiture as punishment had become rare, the use of the death penalty had become less frequent, and prison more frequent, for felonies, although all in all a felony was still a rather serious crime (with prison sentences usually being lengthy). Many of the things we consider felonies today would not have been considered anything more than a high misdemeanor back then while others would have not been considered crimes at all. When the Constitution was ratified, corruption of blood as a punishment was completely taken off the table.
What led to the current definition of felony as being any crime punishable by statute by a fine exceeding X or a prison term greater than a year's length was the departure from the Common Law and the codification of law as was being done in Continental Europe (such as France under Napoleon) at the time.