The pilot reported that while conducting banner towing operations, after the airplane passed through the banner pick up gates, he applied full throttle and pitched the nose of the airplane up. Once the airspeed reached 40 miles per hour (MPH) he looked back to ensure that the banner pickup was successful. The pilot stated that when he turned back around the airspeed had decreased, simultaneously the banner began "pulling", which led to an increase in drag, and the airplane exceeded its critical angle of attack. The airplane aerodynamically stalled, descended, and impacted the ground. The airplane impacted the ground left wing first, and sustained substantial damage to both wings and the fuselage. The pilot reported there were no pre-impact mechanical failures or malfunctions with the airframe or engine that would have precluded normal operation.