Katherine Jackson claims AEG Live were neglectful when they hired doctor Conrad Murray, who gave the singer the overdose that lead to his death in 2009.
Murray is currently in jail for accidental manslaughter.
AEG Live claim the figure is “preposterous”.
Michael Jackson’s death was caused by an overdose of Propofol, a drug used as a sedative to help the singer sleep.
He died three weeks before the start of his 50-show comeback tour at the London 02 Arena. Katherine Jackson believes rehearsals for the tour pushed her son too hard.
AEG Live say the singer had a long history of drug abuse before he met Murray.
The promoters also say they were not responsible for the hire or supervision of the doctor and that the $40bn in damages sought is a “preposterous” amount considering Jackson’s ”descending career”.
Conrad Murray might be asked to give evidence at the trial, but he has the right to refuse. The trial is due to start this week.