使用QMAKE构建项目的32位和64位版本

I need to generate a 32 bits version of my application however I am compiling on a 64 bits OS. I'm looking for a way to make QMake to generate both 32 and 64 bits versions of my application. If this is not possible I would like to know how to switch to 32 bits. I would also like to avoid having to mess with the generated makefile.I need to generate a 32 bits version of my appl