Also known as device independent bitmap (dib), Windows bitmap been around for as long as Windows 3.14. Flash/Flex currently does not support this format. So might you consider my implementation?
My Windows Bitmap classes support bit depth 1, 4, 8, 24 bits per pixel for decoding as well as encoding. It has been tested with Windows XP Paint utility.
For source code, example use and construct, try it out in the ImageDencoder AIR project.