He was a 30-year-old staff artist for Fawcett Publications when he was tapped to do the job. According to his own words, he never really thought of cartooning as serious business. But he did it with a professionalism that was expressed in his writings as the "Crusty Curmudgeon" in later years.
His deceptively simple style was unsuccessfully imitated by a series of artists for Fawcett in the early years, until Beck's shop became the sole source of Captain Marvel art, and eventually the house style was cleaned up ad a certain stability in quality level achieved.
I have found it hard to give a fair assessment of the full arc of Beck's art development for the character, considering the scarcity of reprints available until recently and the prohibitive pricing of the original comics. Most of the reprints DC made available were originally drawn in 1945 or later, by which time the style and quality level was pretty much set. But the "Archive Editions" DC put out showed every single Captain Marvel story from the first couple of years of his publishing history.
The earliest stories were kind of rough. Beck's art was deliberately simple. He was a fan of such newspaper strips as Little Orphan Annie and Barney Google, and it shows in the charicaturistc renderings of certain characters. Captain Marvel's face was said to be based on Fred MacMurray,and was pretty well-defined from the beginning, but Billy Batson was little more than a small mouth, a small nose, and two dots for eyes. Figures seemed awkwardly positioned, layouts rough and simplistic. Some stories, like the origin of Dr. Sivana, showed Beck at the top of his craft, but others must have been done in a mad haste or by staffers.
When at the top of his game though, as he was after those first few years, a close examination of his work reveals an undercover sophistication and depth that is truly underappreciated by many comics fans today.
Clarity was his watchword, and he believed that everything on the page should advance the story. Thus, nothing that didn't advance the story should be on the page. This meant that excessive detailing, shadows, wrinkles, etc, were not necessary. The linework was always clean, not sketchy. Sometimes there was no background at all. But if you took a moment to look, you would notice that the perspective, very deliberately chosen, was drawn with precision. Little details, like a telephone on a desk, grain in wood, or a wrinkle in a carpet would give the image an touch of authenticity. There would be a key shadow under an armpit or highlights on the golden wristbands of the hero that gave the figures depth.
Landscapes would show true depth of field. Longshots used to establish a scene would show details like certain types of trees that would indicate the location. A long shot over the ocean would include cloud formations. A sunset might show silhouettes of palm trees.
And oh, those clouds and smoke! If ever Beck had to draw smoke from a fire or the clouds around a bolt of lightning, the billows would roil like a Wagnerian opera!
Space ships and fortresses would have a Flash Gordon/Buck Rogers art-deco style that gave them a fantastical, yet believable appearance.
His anatomy was impeccable.With just a few lines he could establish a person's figure and character from any angle. Ofttimes Captain Marvel would be shown flying, with his feet in the frame, and the feet were always drawn perfectly, with appropriate perspective. The difficulty of this was highlighted by the awkward, inexpertly drawn feet of other artists that drew attention to themselves like a blemish on the page. But Beck's feet were so perfectly drawn that they almost disappeared.
His animal anatomy was likewise fully believable. He was even able to show a gradual evolution of Mr. Tawny, the talking tiger, evolve from his natural, four-legged stance to his anthropomorphic, two-legged stride in just a few pages. The effect was so subtle you wouldn't even think about it.
Note ease of following the story left-to-right, top-to-bottom, use of perspective, expressive gestures, anthropomorphic animal anatomy, judicious choices of how much background and foreground elements, use of silhouette and shadow, and juxtaposition of close-up, medium, and long shots.
After the 1953 shutdown of Fawcett's comics line. Beck never returned to comic full-time except for a brief revival in 1966 and his return to Captain Marvel in the 1970's But his legacy had been established through those 13 years of Captain Marvel and the Marvel family, and even still is underappreciated for those little things that made his art so perfect.