David being classified an underdog doesn't necessarily mean he wasn't actually at an advantage compared to Goliath in some ways, just that it appeared that way. In fact if you read the biblical passage it seems to paint the picture that David was actually extremely skilled, and he claims to have killed bears and lions before while defending his sheep. It's the fact that it's a shepherd boy taking on a warrior giant that makes in an underdog story, not the skill with which David wielded his weapon.

Malcolm Gladwell's book David and Goliath talks about that example (among others). It's a pretty interesting read.