Was the gospel of Mark really written first?

There are many scholars today that will state as fact that Mark was written before Matthew by somewhere around 15-20 years.  This idea also states that Mark was the source material for both Matthew and Luke.  The sources used to justify this come from Eusebius who supposedly quotes early church fathers as stating such.  However, according to tradition Matthew was always considered to be the first written gospel by most of the churches.  This is why it was placed first in the New Testament.  Eusebius was a heretic that believed in many false doctrines and quite a bit of his information has been shown to be misquoted or complete lies.  Those he is truthful in recording use their own ideas instead of what the Bible clearly teaches.  Also there is not any real evidence that the gospel writers used each other as source material (this is only suggested because of the similarities between the first 3 gospels).