This article contains spoilers for Avengers #36.
Black Panther's claim to Mjolnir is actually older than Thor's. Everyone knows the story of Thor's enchanted hammer, Mjolnir. It can be wielded only by those who are worthy, a subjective term that appears to be linked to a willingness to sacrifice yourself on behalf of others.
During his seminal Mighty Thor run, writer Jason Aaron rewrote the history of Mjolnir. He revealed that millennia ago, Asgard had almost been ravaged by a cosmic force called the God Tempest. "It began with the first wind that ever howled down out of the void," the librarian of all the gods explained, "and it had been growing in size and fury with each subsequent eon. They called it the God Tempest. The Mother of Thunder." Odin successfully trapped the God Tempest in a chunk of Uru, which he had forged into Mjolnir. It was only when he proved unable to safely use Mjolnir himself that Odin placed the worthiness enchantment upon the hammer, an enchantment that has grown more powerful with time, bound to the will of the God Tempest itself.
In a remarkable twist, this week's Avengers #36 reveals Black Panther is one of the few people considered worthy by Mjolnir. Jason Aaron's current Avengers run has been carefully weaving his Thor lore into the wider history of the Marvel Universe, revealing the first Avengers were formed 1,000,000 years ago. The lineup included Odin himself, as well as the first Black Panther. And, in a flashback scene, the issue reveals this first Black Panther wielded Mjolnir. Ever since, the hammer has considered every single Black Panther worthy.
It's a wonderful scene, and amusingly it means Black Panther's claim to Mjolnir actually predates Thor's. The first Black Panther wielded the hammer long before the worthiness enchantment was cast upon it before Thor was even born. What's more, the God Tempest seems to have approved of him, which must surely have driven Odin the All-Father wild with fury. The idea the God Tempest would approve of a mere human, and yet refuse to bend to the will of Odin the All-Father, would appall him. Indeed, this might explain why he ultimately placed the worthiness enchantment upon the hammer and left it in the chambers of Asgard, until it was eventually claimed by Thor Odinson.
The Black Panthers, of course, are chosen by the Panther God Bast. It seems Bast and the God Tempest have the same view of what makes a person worthy - and King T'Challa, and all his predecessors, ticked the box for them both.