The History of WLS ... 
WLS was originally WJR. The call letters WLS were granted by the FCC on April 11, 1924. The Big 89 actually started out at 870 am.  The 50,000 watt blow torch operated at just 500 watts and was licensed to Crete, Illinois. The legendary WLS call letters stood for World's Largest Store for WLS owner Sears, Roebuck & Company. For a time WLS was owned by ABC,  not the ABC, but the Agricultural Broadcasting Company. On March 29, 1941, WLS moved to 890 am. For a long time WLS shared the 890 frequency with WENR. WENR was for many years part of the NBC Blue Network. When the government said NBC had to split, WENR became ABC for Chicago. When ABC bought WLS, the WLS call letters prevailed. In May of 1960 WLS became a subsidiary ABC.  WENR was also ABC's first set of TV call-letters in Chicago although eventually the WLS call letters were used on tv as well. Since 1960, WLS has been owned by ABC, the American Broadcasting Company, ABC/Capital Cities Broadcasting, and now Disney. WLS joined the Top 40 ranks on May 2, 1960.  With the playing of "Ally Oop" by the Hollywood Argyles, WLS would never be the same. WLS remained one of the Nation's greatest top 40 stations until August 23, 1989. After ending with Chicago's "Just You and Me",  Musicradio WLS was no more. Today, WLS is News Talk 89.

There is some confusion as to the first set of call letters for WLS.  Here are 2 conflicting articles. ...

From Jeff Miller's Collection...
New Station in Chicago
This article appeared in the New York Times on April 6, 1924 

Agricultural talks will make up the bulk of the programs to be broadcast from the new station that the Herald and Examiner and Sears-Roebuck plan to open in Chicago this month. The call will be WBBX and the wave length 448 meters. Farm news, agricultural lectures by university professors, questions and answers about crop conditions in all segments of the country, and music and stories intended to appeal to farm children will be the main feature of the program. 

[Jeff Miller note: When this station signed on the air, rather than using the  sequentially-assigned call WBBX, it used the call WLS. Requested calls became common in mid-1924. 

From Jeff Miller's Collection... 
A Chronology of AM Radio Broadcasting Part 2: 1923-1960...

"Apr. 12, 1924. WLS Chicago IL formal dedication. [According to Prairie Farmer and WLS, Sears originally did a farm program beginning March 21, 1924, with its first assigned call letters, WBBX, from the studios of WMAQ. Its first test program from the Tower studio was on April 9 using the call WES [World's Economy  Store]. The next two evenings (April 10 & 11) they aired more test programs. On the evening of formal dedication Sears changed the call to WLS.]

From Jeff Miller's Collection & The New York Times, June 1, 1924...

"New Radiophone Stations Operating in Chicago"
Several changes have been made in Chicago broadcasting stations.  Station WLS is the Sears-Roebuck Agricultural Foundation.  The call letters, WLS, have been substituted for the earlier ones, WJR, by the government because of regulations recently passed concerning stations of this type. The wave length has also been changed from 448 meters to 345 meters.  A train whistle has been adopted as the signal opening a program and signing off."

From The Man from Mars Website...
   Here is a QSL card from WLS in 1971. it includes information on WLS' ownership.