Authorities said the wingtips collided after both planes pushed back from their gates, the Chicago Tribune reported Thursday.
United spokeswoman Megan McCarthy said no one was injured in the incident.
"One of the aircraft started moving to go toward the taxiway, and they touched wingtips," McCarthy said. "The pilots put safety first, and both aircraft immediately returned to the gates."
McCarthy said all of the passengers from both planes were accommodated on other flights.
She said a maintenance team is assessing the damage to a planes and a full investigation into the collision has been planned.