A process in which force is generated within smooth muscle tissue, resulting in a change in muscle geometry. This process occurs in the proximal stomach. Force generation involves a chemo-mechanical energy conversion step that is carried out by the actin/myosin complex activity, which generates force through ATP hydrolysis. The proximal stomach, composed of the fundus and upper body, shows low frequency, sustained tonic contractions that are responsible for generating a basal pressure within the stomach.