How to save image as 32-bit BMP?

Photoshop >> Menu >> File >> Save As >> Choose format BMP in dropdown box >> Save button >> Choose 32-bit format >> OK button

However, photoshop will save the image as XRGB. In case the image has alpha channel (as ARGB), must use another tool:
http://www.qualibyte.com/pixelformer/

In Pixelformer, choose Menu >> File >> Export, Choose export file type as .bmp, Click 'Save', the following BMP options dialog should be shown, choose 32bit ARGB: