Rapidly developing a customizable mobile application and the related software as a service (SaaS) is challenging and rarely studied. Traditionally, SaaS solutions are mainly accessed using personal computers, but the mobile SaaS solutions are needed in the tourism sector, for example, where users are mobile. This paper presents a case study where the original need was to design a customizable mobile tourism guide service for use by several small tourism companies, and to assess its functionality in a field study. The result of applying the Vaadin 6 Java web framework and LAMP technologies was a robust mobile application SaaS prototype system that fulfilled the essential design needs in the eight field test cases. This study shows that the field testing of a mobile concept can be completed easier when using Vaadin Java web framework, as it provides support for cross-platform functionality and GUI design, and completes, for example, LAMP-based SaaS solution. However, results point it out that new digital navigation features were needed to develop or improved and mobile web approach causes some usability challenges especially in the compass based navigation and user tracking. This study provides an example of how to develop a SaaS-based mobile service prototyping environment, which is needed while field testing new B2B mobile services with various groups of stakeholders. Our case study analysis reveal that the Vaadin development environment facilitates the rapid prototyping for digital services in an affordable way. The overall contribution of this paper is predominantly for software engineers and web application developers.