Over a decade, many bug assignment methods have been proposed in order to assist developers to read bug reports submitted daily and numerously, and to assign an appropriate developer. However, they tend to concentrate their assignments on a small number of particular developers. Applying the methods to the projects which have releases would reduce the number of bugs that developers can fix by the next release date because the time that developers can devote to bug-fixing is limited. In this study, we propose the release-aware bug-fixing task assignment method to mitigate the task concentration and increase the number of bugs that developers can fix by the next release date. This method employs mathematical programming to find the best combination at project level while the traditional methods find the best pair of a bug and a developer (at individual level).