Many Software-Defined Networking (SDN) controllers support a 'northbound interface' by which applications can interact with the SDN controller and (indirectly) control the underlying SDN network. The absence of a standard for the northbound interface of these controllers makes it difficult for application developers to create interoperable/portable code (i.e., code that works with any SDN controller). Developers are forced to re-write almost all of their code every time they want to support a new controller. This tedious and time consuming process is typically a start-over software development cycle that involves learning new APIs, data models, and controller-specific conventions. In this paper, we present RAPTOR, a REST-based API translaTOR service for SDN networks that allows users to develop their network control software independent of any particular SDN controller. RAPTOR exposes its own REST-based API functions and data models to user applications and translates application requests into controller-specific northbound interface calls. To demonstrate the viability of RAPTOR, we implemented and deployed RAPTOR in GENI and used it in conjunction with different types of controllers. We also developed GENI Desktop modules that interacted with RAPTOR to install, list, delete and monitor end-to-end flows regardless of the controller used for controlling the underlying switches.