First Appearance: Invaders #7 (July 1976).
Appearances: Invaders #7, 11-12, 34, 39-41, Captain America #253-254, Namor #12-13, 15, 19, 21, 23, Namor Annual #1, Marvel Universe #1, Marvel Comics Presents #89, Marvel Super-Heroes v3 #4, Mighty World of Marvel #8 (mentioned; see the Note below), Citizen V & the V-Battalion v2 #1.
Years Active: 1942-present.

Jacqueline Falsworth was the sister of Brian Falsworth (aka Union Jack (II)) and the daughter of Montgomery, Lord Falsworth (aka Union Jack (I)). In 1942 in England Jacqueline befriended the Invaders. Unfortunately, Jacqueline became a little too entangled in the Invaders' affairs and was attacked and bitten by the vampire Baron Blood. Jacqueline was dying and need a blood transfusion to save her life, so Invaders member Human Torch (I) gave Jacqueline the requisite blood transfusion.

The combination of Baron Blood's bite and the blood of the Torch gave Spitfire superspeed. She then joined the Invaders and fought against the Axis during the war. At the end of the war she retired from crime- and evil-fighting, and over the next few years lost her powers, or so she said (see Note below). In 1953 she was shown attending the funeral of her brother in 1953.

Very recently her youth was restored, as were her powers.

Note: Mighty World of Marvel #8 referred to a "Lady Farnsworth" who was a wealthy old woman who lived in an "old stately home" and who was a "super-woman back in WW II." It could be argued, of course, that there was a whole other character named Lady Farnsworth who fought during The Big One as a superheroine, but Occam's Razor dictates that it is Spitfire being referred to here and that the letterer for MWOM #8 simply screwed up.

Note: Donald Campbell points out that

In Marvel Super-Heroes (vol. 3) #4, it was revealed that, contrary to what she once told Captain America (I), Jacqueline Falsworth-Crichton had actually NOT lost her powers in the years following World War Two and that she still had them, at somewhat diminished levels, when she was an old woman. Although her recent rejuvenation also restored her full powers, she has not, as yet, resumed her crime-fighting activities.