Dell Publishing was founded by George T. Delacorte Jr. in 1921. For the first eight years Dell published pulp magazines. In 1929, Dell Publishing began publishing The Funnies, described by the Library of Congress as "a short-lived newspaper tabloid insert". Each insert measured 10.5" x 15.5". The magazine ran 36 weekly issues, published Saturdays from January 16, 1929, to October 16, 1930. The cover price rose from 10¢ to 30¢ with issue #3. This was reduced to a nickel from issue #22 to the last issue.
TheComicBooks.com describes the content of these early issues:
Noted comic historian, Ron Goulart, describes this early entry into “modern comic books” as more of a glorified Sunday comics section:
In Goulart’s “The Comic Book Reader’s Companion” he expands on the introduction of “The Funnies”:[i]
In addition to the 36 issues of The Funnies, Dell also published Clancy the Cop (2 issues, 1930), Deadwood Gulch (1 issue, 1931), and Bug Movies (1 issue, 1931).
Format would seem to be the key discriminator between inclusion in either the Platinum or Golden age. Don Markstein’s Toonopedia describes the introduction of Popular Comics, which published reprint material in the “American Comic Book” format: 
I find it interesting that many comic historians consider Famous Funnies (Eastern Color Printing Company, 1933) as the first modern comic book. Famous Funnies was first introduced as a Proctor and Gamble giveaway with reprinted newspaper comics for content. About a year later, Famous Funnies: A Carnival of Comics was sold on the newstand for 10¢. Albert Ching opines in a blog
The elimination of The Funnies as the first modern comic book is apparently based on the use a tabloid format on newsprint. Despite its original content and news stand distribution, The Funnies that Dell published in 1929 just doesn’t look what is considered to be a comic by today’s standards. This is also the case for Comic Monthly, a magazine sized comic that was distributed on newsstands in 1922, predating The Funnies by seven years. Goulart’s description of this early newsstand comic certainly sounds similar to the size and format of Famous Funnies:
The Funnies #1 (January 16, 1929)
Popular Comics #1 (February 1936)
Returning to the question: “What was the first Dell comic?” If you ignore the eras and format distinctions, The Funnies #1 (1929) is the first Dell comic. If you limit your consideration to the Golden Age of comics and restrict yourself to the modern comic book format, then the first Dell comic is Popular Comics #1 (Feb 1936).
 Putting Dell on the Map, William H. Lyles, Greenwood Press, 1983, ISBN 0-313-23667-4
 Goulart, R. (1986). Ron Goulart’s Great History of Comic Books, Contemporary Books, Chicago, pg. 2-3.
 Ching, A. (2010). “What Was the First Comic Book?” Life's Little Mysteries Contributor, 08 June 2010 9:03 AM ET, http://www.lifeslittlemysteries.com/what-was-the-first-comic-book-0838/
 Goulart, R. (1986). Ron Goulart’s Great History of Comic Books, Contemporary Books, Chicago, pg. 1.