Can you assign combat damage to a planeswalker without attacking it directly?

When an unblocked attacking creature would deal combat damage to the defending player, that player's opponent (the attacker) may instead redirect some or all of that damage to a planeswalker the defending player controls. This is covered under CR 306.7 and the combat damage assignment rules in CR 510.1–510.2.

The key distinction: you do not have to declare the planeswalker as the attack target. The creature attacks the player (or the planeswalker directly), becomes unblocked, and then during combat damage assignment the attacking player chooses to have the damage hit a planeswalker instead of the player. This is a special exception to the normal rule that damage goes to whoever or whatever was attacked.

If a creature is blocked, its combat damage is assigned to the blocking creatures in the normal way—it cannot be redirected to a planeswalker in that case. Redirection only applies to damage that would otherwise be dealt to the defending player by an unblocked creature.

Concrete example: Your 5/5 Grizzly Bear attacks your opponent, who controls Chandra, Torch of Defiance with 4 loyalty. Your opponent has no blockers. During the combat damage step, you choose to redirect all 5 damage to Chandra instead of your opponent. Chandra takes 5 damage, loses 5 loyalty counters, and is sent to the graveyard—even though you never declared her as an attack target.

Note that some effects (like Planeswalker protection spells or abilities that prevent redirected damage) can change this outcome, so always check for any active replacement or prevention effects (CR 614.1).