Dear PJ Smith,
Thank you for posting your request on History Hub!
If you have not done so already, you may wish to request your relative’s Official Military Personnel File (OMPF). OMPFs and individual medical reports for soldiers of the U.S. Army who served during World War II and who separated from the service prior to 1958 are in the custody of NARA's National Personnel Records Center in St. Louis. In many cases where personnel records were destroyed in the 1973 fire, proof of service can be provided from other records such as morning reports, payrolls, and military orders, and a certificate of military service will be issued. Please complete a GSA Standard Form 180 and mail it to NARA's National Personnel Records Center, (Military Personnel Records), 1 Archives Drive, St. Louis, MO 63138-1002. You can also fax the form to 314-801-9195 OR view the record by visiting the NPRC Archival Research Room in St. Louis, MO. For more information see Official Military Personnel Files (OMPF), Archival Records Requests.
We searched the World War II Operations Reports, 1940-1948 in the Records of the Adjutant General's Office, 1917-1981 (Record Group 407) and located histories, after action reports, and other documents of the 99th Infantry Battalion filed in Box 16923. We also searched Unit Annual Reports, 1940 - 1949 in the Records of the Office of the Surgeon General (Army) (Record Group 112) and located a file for the 99th Infantry Bn. For access to and/or copies of these records, please contact the National Archives at College Park - Textual Reference (RDT2) via email at email@example.com.
In addition, we searched National Catalog and located the series Moving Images Relating to Military Activities in the Records of the Office of the Chief Signal Officer (Record Group 111) that includes a film depicting them as described in the Shot List Card for ARRIVAL OF GEN. BRADLEY AND PARADE OF ALLIED FORCES, OSLO, NORWAY. For access to and/or copies of these records, please contact the National Archives at College Park - Motion Pictures (RDSM) via email at firstname.lastname@example.org.
There is a brief mention of the 99th Infantry Battalion on page 245 of Volume 1 - "War Report: Office of Strategic Services: Washington Organization" in the Records of the Office of Strategic Service (Record Group 226) that has been digitized and can be viewed online through the Catalog.
The Library of Congress’s Veteran’s History Project includes materials from veterans of the unit, some of which have been digitized and may be viewed online. We also searched World Cat and located several books and other materials pertaining to the 99th Infantry Battalion. We also located multiple sources on Google Books. You also may be interested in the website of the 99th Infantry Battalion Educational Foundation.
We hope this is helpful. Best of luck with your family research!