First Appearance: Mystic Comics #6 (October 1941).
Golden Age Appearances: Mystic Comics #6-10, Mystic v2 #1, All-Winners #2, USA Comics #6-14 & #16-17, Kid Komics #4-6 & 9-10, Daring Comics #11-12.
Modern Appearances: Invaders #18-19, 26, 34, Midnight Sons Unlimited #5, Thunderbolts #40, 41, 45-51, Citizen V & the V-Battalion v1 #1-3, v2 #1-4.
Years Active: 1941-present.
The Destroyer has had a somewhat complex history, so bear with me, 'cause this gets complicated.
Originally, during the Golden Age, the Desteroyer was Keen Marlow. Marlow was an American journalist who'd gone inside Nazi Germany to investigate the horrors of that regime and report back to his States-side newspaper. Unfortunately, Marlow was a better reporter than investigative journalist (or, to put it less politely, spy), and he was caught snooping by the Germans and thrown into a concentration camp. (Yes, they used that phrase, although in mid-1941, when Mystic Comics was written, it didn't have the horrific connotations it has today.)
In the camp Marlow meets fellow prisoner Professor Eric Schmitt, a German scientist who'd refused to cooperate with the Nazis and was imprisoned for his troubles. The dying Schmitt gives Marlow a formula he's created himself: a version of the Super-Soldier Serum which would later empower Captain America (I). The Serum works on Marlow, giving him the same speed-strength-endurance-prime-of-human-potential which Steve Rogers would later put to such good use as Cap. Schmitt, unfortunately, dies just then.
The newly empowered Marlow breaks out of the camp, avenges Schmitt's death, and goes on a rampage behind enemy lines, working as the costumed Destroyer. He ranged across Europe and even Japan, although mostly he worked in Germany. At times he was aided by the Allied agent Florence von Banger. And when the war ended and the Golden Age drew to a close, the Destroyer disappeared.
Then, during the 1970s, the Destroyer was brought back. Sorta. Kinda. Roy Thomas, for reasons I've never seen explained, decided to replace Keen Marlow with with someone else. The Invaders heard tell of someone operating behind enemy lines, wearing the Destroyer's costume and calling himself "the Mighty Destroyer." On investigation, however, it was revealed that the man underneath the Destroyer's mask was Brian Falsworth, the brother to the heroine and Invaders member Spitfire. Falsworth was moreover the son of Montgomery, Lord Falsworth, who had been active during World War One as the Union Jack (I).
Brian Falsworth had gone through the same experiences as Keen Marlow, having gone into Germany to investigate the horrors of the Nazi's rule and been imprisoned next to Professor Eric Schmitt. Schmitt had given Falsworth a version of the Super-Soldier Serum, this time chronologically after Captain America (I) got his Serum dose. Falsworth, like Keen Marlow, escaped from the camp and waged a guerrilla war on the Germans as the costumed Mighty Destroyer. (When questioned about "Keen Marlow," Falsworth dismissed the name as fictitious, a move I thought and think was tacky on Roy Thomas' part.)
Falsworth eventually put aside the costume of the Destroyer and became Union Jack (II). Falsworth's close friend Roger Aubrey, who had been the Dyna-Mite, put on the Destroyer's costume and began calling himself the Mighty Destroyer. (So, in a sense, Roger Aubrey is Destroyer (III).) For the rest of the war Aubrey served as the Destroyer, fighting evil and, in January 1945, the Iron Cross alongside the Blazing Skull.
Still later, in the pages of Thunderbolts and the two Citizen V miniseries, still more of Aubrey's past is revealed. Aubrey and Brian Falsworth had been lovers. Aubrey, after the war, was a founding member of the Penance Council, the leaders of the V-Battalion, and had served on the Council since the war ended. At some point between WW2 and the present Aubrey had traveled back to the "Stone Age," which he found to be an "unpleasant place." After the events of the second Citizen V miniseries Aubrey retired from the V-Battalion.
Note: In the History of Marvels comic Keen Marlow's name was invoked, as the fictitious identity used by the "real" Destroyer during the war. So at least that much of the one, true Destroyer still exists in the MU.