Captain Mitchell was a member of the Merchant Marines and the captain of the Achilles, the ocean freighter that transported the Giza Stargate from Africa to the US in 1939.
Mitchell was a sea captain stationed out of Boston Harbor. During the early days of World War II, the Achilles was tasked with transporting a large crate housing Professor Langford's Stargate from Africa to Boston. None of the crew, including her captain, knew the true nature of the cargo. However, the captain took his job seriously and made an indirect course across the Atlantic to avoid German U-boats.
Coincidentally, Mitchell was also the grandfather of Cameron Mitchell, later leader of SG-1, the flagship Stargate team. The two Mitchells bore an uncanny resemblance to each other. Apparently, Cameron never knew his grandfather, but his grandmother showed him a picture he later kept a copy of in his locker at Stargate Command.
When Ba'al attempted to use a time machine to alter history and make himself ruler of Goa'uld and galaxy, the point he chose to make his move was 1939, when Achilles was moving the Stargate in the North Atlantic Ocean. Although Ba'al's crew did overwhelm the Achilles crew in the initial attempt, a mortally wounded Captain Mitchell was able to throw the bomb Ba'al had left behind into the ocean before it detonated, thus saving the Achilles from a watery grave and leaving it adrift in the Arctic Circle.
A quirk of fate kept SG-1 immune from the timeline alterations, and they were able to access the same time machine on Praxyon. Everyone but Cameron Mitchell died in a battle for the device; he escaped to 1929. Ten years later, Cameron stowed away on board the Achilles, ambushing Ba'al when he arrived through the Stargate. Captain Mitchell and his crew were surprised to find a doppleganger of himself, a hole in his hull from the Stargate's kawhoosh, and several dead attackers (Ba'al and his Jaffa). But the timeline was apparently restored with the Achilles reaching Boston, and Stargate program's history otherwise intact. Cameron's locker picture of his grandfather in the restored timeline depicted both Captain Mitchell and his time travelling grandson—although it's uncertain if Cameron ever realized an "alternate" version of himself was the other man in the family photograph.
--Aurora 22:19, 15 November 2008 (PST)