Damage distribution with Banding and Trample
When attacking with creatures that have some combination of Banding and Trample you must still deal lethal damage to all blocking creatures before you can deal damage to the defending player (or the Planeswalker you attacked).
Rule 702.21k explains how banding affects combat damage when attacking. Basically it allows you to ignore the damage assignment order the defending player chooses, and you assign damage to your creatures as you choose. It does nothing for how you assign combat damage to the defending creatures (or players or planeswalkers).
Rule 702.19b explains how trample works. This rule doesn't change the rules for assigning combat damage to creatures, but allows for once all blocking creatures have been dealt lethal damage any remaining damage may be assigned to the player or planeswalker that creature is attacking.
So when you have an attacking band (at least 1 of which has trample), and the band gets blocked by multiple creatures you must assign lethal damage to all blocking creatures before you can trample over any damage.

702.21k During the combat damage step, if a blocking creature is blocking a creature with banding [...] the active player (rather than the defending player) chooses how the blocking creature’s damage is assigned. That player can divide that creature’s combat damage as he or she chooses among any number of creatures it’s blocking. This is an exception to the procedure described in rule 510.1d.
702.19b The controller of an attacking creature with trample first assigns damage to the creature(s) blocking it. Once all those blocking creatures are assigned lethal damage, any remaining damage is assigned as its controller chooses among those blocking creatures and the player or planeswalker the creature is attacking. [...] The attacking creature’s controller need not assign lethal damage to all those blocking creatures but in that case can’t assign any damage to the player or planeswalker it’s attacking.
510.1d A blocking creature assigns combat damage to the creatures it’s blocking. If it isn’t currently blocking any creatures (if, for example, they were destroyed or removed from combat), it assigns no combat damage. If it’s blocking exactly one creature, it assigns all its combat damage to that creature. If it’s blocking two or more creatures, it assigns its combat damage to those creatures according to the damage assignment order announced for it. This may allow the blocking creature to divide its combat damage. However, it can’t assign combat damage to a creature that it’s blocking unless, when combat damage assignments are complete, each creature that precedes that blocked creature is assigned lethal damage. When checking for assigned lethal damage, take into account damage already marked on the creature and damage from other creatures that’s being assigned during the same combat damage step, but not any abilities or effects that might change the amount of damage that’s actually dealt. An amount of damage that’s greater than a creature’s lethal damage may be assigned to it.