Was David a man of war before he fought Goliath?
Yes
He was a mighty, valiant man of war.
Then answered one of the servants, and said, Behold, I have seen a son of Jesse the Bethlehemite,
that is cunning in playing, and a mighty valiant man, and a man of war, and prudent in matters, and a comely person,
and the LORD is with him. 1 Samuel 16:18
No
He was "but a youth."
And Saul said to David, Thou art not able to go against this Philistine to fight with him: for
thou art but a youth,
and he a man of war from his youth. ... And Saul armed David with his armour, and he put an helmet of brass upon
his head; also he armed him with a coat of mail. And David girded his sword upon his armour,
and he assayed to go;
for he had not proved it. And David said unto Saul, I cannot go with these; for I have not proved
them. And David put
them off him. 1 Samuel 17:33-39