The right seat pilot reported that after performing a preflight inspection, he anticipated the airplane would need to be refueled and subsequently left the fuel caps unlocked. After a discussion with the left seat pilot (who was acting as the pilot-in-command), they decided they would not need to refuel before the flight. Both pilots performed another walk around inspection and neither pilot noticed that the fuel caps remained unlocked. Shortly after takeoff, the right seat pilot noticed that both fuel caps were not secure, and that fuel was escaping from both main fuel tanks. He elected to make return to the departure airport and land on the opposite direction runway. The pilots were both manipulating the controls when they landed the airplane “hard” on the main landing gear. The airplane then bounced and veered to the right of the runway, impacting a terminal sign and taxi light. The hard landing and subsequent runway excursion resulted in substantial damage to the wings and fuselage. The pilots reported that there were no preaccident mechanical malfunctions or failures with the airplane that would have precluded normal operation.