_WIN32 is not defined for header files

When compiling a project for windows (32 bit mingw) there seems to be a strange issue with the _WIN32 define in header files: in .cpp files the define exists and works, in .h files it is set and valid according to Qt Creator, but when the application is built and runs any statements in a .h file within the block #ifdef _WIN32 will not exist in the executable.

Either it is a bug, or (strange) expected behaviour that _WIN32 is defined for .cpp files and not .h files or Qt creator has a bug whereby it isn't differentiating .h and .cpp macros/defines