Shake Shack is launching its first chicken sandwich nationwide.
The 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 International Airport locations, and stadium locations.
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 had been the chicken dog, a hot dog made with chicken, apple, and sage sausage.
Shake Shack has only 75 restaurants, though, compared with Chick-fil-A's more than 1,800 locations.
Before 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 Thursday in a news release.
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's 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.
Related: See Shake Shack locations around the world:
Shake Shack locations
Shake Shack just fired a shot at Chick-fil-A
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A logo sits on display outside the first Shake Shack burger restaurant to open in Moscow, Russia, on Tuesday, Dec. 24, 2013. The New York burger joint known for its queues will sell ShackBurgers for 235 rubles ($7.15), about 50 percent more than in New York and almost triple the price of a local Big Mac. Photographer: Andrey Rudakov/Bloomberg via Getty Images
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WASHINGTON, DC - MAY 31: Photos for FOOD SECTION : For June 8 First Bite review -Exterior of the busy lunch time at the Shake Shack in Washington, D.C. on May 31, 2011 (Photo by Marvin Joseph/The Washington Post via Getty Images)
A logo hangs on a sign outside Shake Shack's new burger restaurant in London, U.K., on Tuesday, July 2, 2013. Shake Shack, opening in London's Covent Garden this week, started as a hotdog cart in New York's Madison Square Park, and has outlets in six U.S. states as well as in the Middle East and Turkey. Photographer: Simon Dawson/Bloomberg via Getty Images
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NEW YORK, NY - JANUARY 30: Free Shake Shack hamburgers are given out outside of the New York Stock Exchange (NYSE) during the burger company's IPO on January 30, 2015 in New York City. Hamburger chain Shake Shack rose more than 130 percent in its trading debut on the NYSE Friday. Shares for the New York based burger chain opened at $47, and quickly climbed above $52 before dipping back to $48.77 for a 132 percent advance. (Photo by Spencer Platt/Getty Images)