399th may refer to:

• 399th Bombardment Group, inactive United States Air Force unit
• 399th Bombardment Squadron or 99th Air Refueling Squadron, part of the 6th Air Mobility Wing at Birmingham Air National Guard Base, Alabama
• 399th Fighter Squadron or 57th Weather Reconnaissance Squadron, inactive United States Air Force Reserve squadron

• 399 (number)
• 399, the year 399 (CCCXCIX) of the Julian calendar
• 399 BC
• All pages with titles beginning with 399th
• All pages with titles containing 399th

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