The growing field of mHealth has often dealt with the modification of harmful behaviors, such as smoking, that are associated with medical conditions. Smoking behavior has been targeted by numerous mHealth smoking cessation interventions with the help of a wide range of behavior change support (BCS) techniques. However, the exploitation of the established BCS technique of social comparison by mHealth research on smoking cessation has been limited. Based on up-to-date BCS theory and following a user-centered design, we have developed a novel smartphone application, namely QuitIT!, for smoking behavior modification with the help of social comparison. This paper presents the development of QuitIT! as well as its preliminary evaluation through a small pilot study. The latter has yield encouraging initial results concerning the feasibility and the effectiveness of QuitIT! as an mHealth tool for smoking BCS.