Type: Bitmap
    Extension: BMP
      Version: 
  Compression: RLE
  Color Depth: 1- to 24-bit
   Maintainer: Microsoft Corporation
Specification:

BMP is the native bitmap file format of the Microsoft Windows environment.
It efficiently stores mapped or unmapped RGB graphics data with pixels 1-,
4-, 8-, or 24-bits in size. Data may be stored raw or compressed using a
4-bit or 8-bit RLE data compression algorithm. BMP is an excellent choice
for a simple bitmap format which supports a wide range of RGB image data.

There is not single document that is the official &quotBMP Format Specification&quot.
Instead, BMP information is spread over several programming references. You 
can search the Microsoft Developers Network CD-ROMs and the Microsoft Knowledge
Base (available at ftp://ftp.microsoft.com/ and http://www.microsoft.com/), but your
best source of BMP information lies outside of Microsoft and within the following
references:

  Inside Windows File Formats, Tom Swan, Sams Publishing 1993.
    ISBN 0-672-30338-8 $24.95 softcover, 337 pages and 1 disk (3.5 in.).               
    Order: Sams Publishing, 2201 West 103rd Street, Indianapolis,
    IN 46290

  Luse, Marv. "The BMP File Format," Dr. Dobb's Journal, #219 September
    1994 (Vol 9, Issue 10), pp. 18-22.

  The BMP File Format: Part I, Dr. Dobb's Journal, David Charlap, #228
    March 1995 (Vol. 20, Issue 3).

  The BMP File Format: Part II, Dr. Dobb's Journal, David Charlap, #229
    April 1995 (Vol. 20, Issue 4).

The code for the above issues are available at:

  ftp://ftp.mv.com/pub/ddj/1994/1994.09/
  ftp://ftp.mv.com/pub/ddj/1995/1995.03/

Also have a look at:

  http://www.r2m.com/windev/
    Internet Resources for Windows Developers

And look in the OS/2 Developer Connection SDK for OS/2 BMP information.