Shake Shack just fired a shot at Chick-fil-A

Shake Shack is launching its first chicken sandwich nationwide.

The so-called Chick'n Shack, which took two years to develop, features crispy fried chicken, lettuce, pickles and buttermilk herb mayo between two toasted buns.

Starting Thursday it will be available for $6.29 at all Shake Shack locations except for New York City's Theater District location, the JFK Airport Shacks and stadium Shacks.

By making the Chick'n Shack a permanent menu item, Shake Shack is taking direct aim at Chick-fil-A, the leader in the fast-food chicken market.

The popular Atlanta-based chain recently surpassed KFC to become the No. 1 chicken chain in the US.

The menus at Shake Shack and Chick-fil-A are already pretty similar. Both feature sandwiches, french fries, and milkshakes. But the only menu item containing chicken at Shake Shack is the chicken dog, a hot dog made with chicken, apple, and sage sausage.

Prior to its national rollout, the Chick'n Shack was tested this summer at Shake Shack's Brooklyn locations.

Shake Shack culinary director Mark Rosati told Business Insider last year that the sandwich was inspired by the South.

The meat is marinated in buttermilk, which gives it flavor and tenderizes the chicken, he said. There are also pureed vegetables in the buttermilk marinade to give it an extra kick.

The pickles add a vinegar flavor to cut through the richness of all the other ingredients and the lettuce is meant to add some crunchiness, he said.

The herb mayo is flavored with thyme, parsley, chives, and buttermilk to complement the marinade.

"Since we introduced the Chick'n Shack in Brooklyn, guests have been asking us to bring it to all Shacks, and we couldn't be more excited to finally do so," Shake Shack CEO Randy Garutti said in a news release Thursday.

Rosati told Business Insider he came up with the idea of adding a chicken sandwich over two years ago.

"I was hungry for lunch and I didn't want a burger," Rosati said.

Shake Shack wouldn't be the first chain to go after Chick-fil-A's market.

Taco Bell parent company Yum Brands has also been testing a chicken-sandwich chain, Super Chix.

Like Chick-fil-A, Super Chix sells fried-chicken sandwiches, chicken strips, frozen custard, and french fries.