DALLAS — DALLAS – Herb Kelleher, the founder of Dallas-based Southwest Airlines who "revolutionized the industry," has died.

The airline announced his passing Thursday evening.

"We already miss you," Southwest wrote on Twitter with an image that called him "our founder and friend."

Southwest did not say how he died. He was 87 years old.

"His stamp on the airline industry cannot be overstated," Southwest CEO Gary Kelly said. "His vision for making air travel affordable for all revolutionized the industry, and you can still see that transformation taking place today. But his legacy extends far beyond our industry and far beyond the world of entrepreneurship.

"His true impact can only be accurately measured by the hearts and minds of the People who he inspired, motivated, and engaged on a daily basis. And that, I believe, cannot be measured or quantified—but it is to be admired and appreciated by all who have been fortunate enough to experience it. I consider myself blessed to be one of those People.”

Kelleher was an airline industry titan. He built Southwest from a regional Texas airline with just three routes into the largest domestic carrier in the country.

“It’s a sad day for aviation, herb was a pioneer," Dallas' Director of Aviation, Mark Duebner, said in a prepared statement Thursday. "I don’t think Love Field would be what it is today if Herb didn’t have the vision to start Southwest. It’s been a great airline and a great partner to the city for a long time.”

Southwest – first called Air Southwest Co. – was incorporated in March of 1967, according to the company's timeline. The airline began service between Dallas, San Antonio and Houston in June of 1971 after Kelleher implored the Texas Supreme Court to throw out a restraining order filed by major corporations fearing disruption from Southwest's operations.

In his time as the airline's chairman and CEO of Southwest, Kelleher famously implemented a company culture still admired today. Southwest routinely finds itself atop lists of America's best companies to work for, landing at No. 10 on Glassdoor's list for 2019.

"Herb knew all along that 'The business of business is People' – yesterday, today, and forever," Southwest wrote in an online obituary. "Herb decided long ago that our internal Customers, our Employees, would come first."

"[He] is survived by his wife, Joan, three of their four children, many cherished grandchildren, and, of course, his pride and joy, the Employees of Southwest Airlines."

This is a developing story and we will be continuously updating throughout. Check back for new information and don't forget to download our free WFAA app to stay up-to-date on all news stories in the Dallas-Fort Worth area.