Widespread concern has been expressed about the persistent low mathematics achievement of students in the US, particularly for students from low-income and minority backgrounds and students with disabilities. Instructional gaming technology, when designed and fictionalized well, has the potential to improve the motivation and mathematics achievement of students with or at-risk for mathematics difficulties (MD). Despite these potential advantages, the research base is scant for efficacious technology tools in early mathematics. This paper describes development and testing of the NumberShire intervention, and discusses results from feasibility studies in kindergarten, first, and second grade classrooms. Project NumberShire supports the development of in-depth knowledge of whole number concepts for students with or at risk for MD in grades K-2, a focus recommended by mathematics education experts. NumberShire is a browser-based, educational video game in which players build an idyllic fairytale village by learning and applying whole number knowledge in three domains of the Common Core State Standards for Mathematics. To test the initial feasibility and usability of NumberShire, researchers aimed to answer four research questions: (1) Is NumberShire reliably efficient and easy for students and teachers to use?; (2) Are students able to focus on and benefit from mathematics content in the game, rather than being distracted by other features?; (3) Are students operating the game as intended?; and (4) Are students engaged in NumberShire mini-games and activities? In fall 2012, 125 students participated in feasibility testing, 50 of which teachers identified as being at risk for difficulties in mathematics on the basis of student performance on screening measures (e.g., EasyCBM) and other classroom assessments. Classrooms were located in three elementary schools in Oregon and Massachusetts. This study used design experiment methodology and iterative end-user testing trials to examine initial feasibility and usability of the NumberShire intervention. Mixed methods research design was also employed to study initial student learning during the feasibility test and guide program revisions in preparation for a formal, rigorous, small scale RCT pilot study to assess intervention promise. Feasibility data were collected in fall 2012 and spring 2013, from a variety of assessment, game, and interview activities. In winter 2013, research staff summarized data from all observations and interviews conducted in second grade classrooms. Results from the first half of the feasibility study suggest second grade students were engaged in and able to use NumberShire with general ease. Second grade teachers perceived the NumberShire intervention as being aligned with important skill objectives and found the features of the intervention useful. Preliminary results suggest NumberShire may support student achievement for second grade students working to demonstrate proficiency with whole number concepts aligned with the Common Core State Standards for Mathematics (CCSSO 2010). Tables are appended.