Following a 78 nautical mile cross-country flight the pilot forgot to extend the airplane's landing gear before landing. As the airplane veered off the runway the left wing tip impacted the ground resulting in substantial damage. The airplane came to rest in an upright position and the pilot was able to exit the airplane unassisted. The pilot was fatally injured four days later in a separate airplane accident; therefore, a NTSB 6120.1 form was not submitted.