A young man with no military experience vs. a giant who's killed every soldier in the army who's attempted to fight him qualifies as an underdog story in my book. Also I'd assume archery skill doesn't really transfer very much to using a sling
I heard an interesting segment on NPR one or two years ago that delve into "David vs Goliath" and they made the point that we have the situation completely backwards.
Some scholars believe that Goliath suffered from a genetic disorder that may have left him rather "simple" in intellect. And slingers during that era were vicious killers capable of hitting birds in flight along with other examples of their prodigious skills and lethality.
The segment made the case that David vs Goliath was a stone-cold killer against a relative 'innocent,' it just happened to be David who had the distinct advantage.
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.
