Captain America made his first appearance in Captain America Comics #1 (Mar. 1941).
Captain America has been the alter ego of Steve Rogers, a frail young man who was enhanced to the peak of human perfection by an experimental serum, in order to aid the United States government's efforts to win World War II. Captain America wears a costume that bears an American flag motif, and is armed with an indestructible, boomerang-like shield that can both be thrown as a weapon and used to defend against others' weapons.
Steven Rogers was born in the 1920s in the Lower East Side of Manhattan, New York City, to poor Irish immigrants, Sarah and Joseph Rogers during the Great Depression. Joseph Rogers died when Steve was only a child and his mother, Sarah, died of pneumonia while Steve was a teen. By early 1940, before America's entry into World War II, Rogers is a tall but scrawny fine arts student specializing in illustration, and a comic book writer and artist.
Disturbed by the rise of the Third Reich, Rogers attempts to enlist, only to be rejected due to his physically frail body. His resolution allows him to be noticed by U.S. Army General Chester Phillips and "Project: Rebirth." Rogers is used as a test subject for the Super-Soldier project, receiving a special serum made by "Dr. Josef Reinstein", later retroactively changed to a code name for the scientist Abraham Erskine. The name "Erskine" was first used in a Captain America novel The Great Gold Steal by Ted White published by Bantam Books in 1968.
The serum is a success, and transforms the frail Steve Rogers into a perfect specimen—a nearly perfect human being with peak strength, agility, stamina, and intelligence. The success of the program leaves Erskine wondering about replicating the experiment on other human beings. The process itself has been inconsistently detailed: while in the original material Steve Rogers is shown receiving injections of the Super-Serum, when the origin was retold in the 1960s, the Comic Code Authority had already put a veto over graphic description of drug intake and abuse, and thus the Super-Serum was retconned into an oral formula. Later accounts hint at a combination of oral and intravenous treatments with a strenuous training regimen, culminating in the Vita-Ray exposure.
Erskine refused to write down every crucial element of the treatment, leaving behind a flawed, imperfect knowledge of the needed steps. Thus, when the Nazi spy Heinz Kruger killed him, Erskine's method of creating new Super-Soldiers died as well. Captain America, in his first act after his transformation, avenges Erskine. In the 1941 origin story and in Tales of Suspense #63, Kruger dies when running into machinery but is not killed by Rogers; in the Captain America #109 and #255 revisions, Rogers causes the spy's death by punching him into machinery.
Unable to create new Super-Soldiers, and willing to hide the Project Rebirth fiasco, the American government casts the now-powerful Rogers as a patriotic superhero, able to counter the menace of the Red Skull as a counter-intelligence agent. As such, he's supplied with a patriotic uniform designed by Rogers himself, a bulletproof shield, a personal side arm, and the codename Captain America, while posing as a clumsy infantry private at Camp Lehigh in Virginia. He forms a friendship with the camp's teenage mascot, James Buchanan "Bucky" Barnes.
Eventually Barnes learns of Rogers' dual identity and offers to keep the secret if he can become Captain America's sidekick, being trained himself to act as the perfect partner. During their adventures, Franklin D. Roosevelt presents Captain America with a new shield, forged from an alloy of steel and vibranium, fused by an unknown catalyst, so effective that it replaces his own firearm. Throughout World War II, Captain America and Bucky fight the Nazi menace both on their own and as members of the superhero team the Invaders as seen in the 1970s comic of the same name. Captain America battles a number of criminal menaces on American soil, including a wide variety of costumed villains: the Wax Man, the Hangman, the Fang, the Black Talon, and the White Death, among others.
In late April 1945, during the closing days of World War II, Captain America and Bucky try to stop the villainous Baron Zemo from destroying an experimental drone plane. Zemo launches the plane with an armed explosive on it with Rogers and Barnes in hot pursuit. The pair reach the plane just before take off. When Bucky tries to defuse the bomb, it explodes in mid-air. The young man is believed killed. Rogers is hurled into the freezing waters of the North Atlantic. Neither is found, and both are presumed dead. It is later revealed that neither character actually died.
Years later, the superhero team the Avengers discovers Steve Rogers' body in the North Atlantic, the Captain's uniform under his soldier's fatigues and still carrying his shield. After he revives, they piece together that Rogers had been preserved in a block of ice since 1945, surviving in such a state only because of his enhancements from Operation: Rebirth. The block had begun to melt after the Sub-Mariner, enraged that an Inuit tribe is worshiping the frozen figure, throws it into the ocean. Rogers accepts membership in the Avengers, and although long out of his time, his considerable experience both in individual combat service and his time with the Invaders makes him a valuable asset to the team. He quickly assumes leadership, and has typically returned to that position throughout the team's history.