Until 1993 it was possible for a serious scholar to argue that David was as unhistorical as Arthur. That position is now much harder to hold, and the reason is a broken piece of basalt.
The Tel Dan stele, found in fragments in northern Israel in 1993 and 1994, is a victory inscription in Aramaic, set up by a king of Damascus in the ninth century BCE, boasting of killing kings of Israel and of the house of David, bytdwd in the consonantal text. That phrase is the standard ancient Near Eastern way of naming a dynasty by its founder. It establishes that within roughly a century and a half of the traditional date of his reign, a neighbouring kingdom identified the southern royal line by his name.
The reading was contested when the fragments were published, and a few scholars argued for a different word division. That challenge has not carried the field, and the great majority now accept the reading. A second possible occurrence on the Mesha stele from Moab, in a damaged line, is much less certain and should not be leaned on.
What this establishes is narrow and worth stating precisely. It establishes a dynastic founder named David. It says nothing about the size of his kingdom, the empire the biblical narrative describes, or any single event in his life.
The scale is where the real argument now sits. Excavation of Jerusalem's earliest strata has produced a settlement that some read as a modest highland town and others as a capital, and the interpretation of the same walls by different teams has been sharply opposed for thirty years.