Here are the first 3 pages of one of Jack's earliest stories, Blue Bolt # 2 (1940). The story is credited to Joe Simon in the first panel, but it's certainly possible Jack contributed story elements, and the artwork was penciled by Jack and inked by Simon. This is widely acknowledged as the first Simon and Kirby collaboration.
This artwork is quite nice. Straightforward and well-composed. The colors help make all the elements stand out from one another.
One of the things I find interesting about these pages is the lack of background elements. Panels 2, 3, 4, and 6 (below) being good examples. I don't know if comics from that time period simply tended to have sparse backgrounds in general, or maybe by this time Jack still hadn't mastered the complexities of perspective, so he felt simplicity was better than trying to create elaborate architecture.
Fun to see Jack experimenting with the green smoke as the lady appears. The unrealistic shadow on the front of the face of the man in panel 3 shows Jack still hasn't developed a confident three-dimensional rendering style. The images are fairly flat and there is a lack of contrast -- for example the shadow on the old man's face in panel 6 really stands out, so the additional shadow he casts behind him doesn't have a logical light source. Right now Jack seems to be using mainly comic book cliches instead of images taken from real life or experience.Not nearly as polished as Jack's later work, but you can see his distinctive style slowly emerging in these early pages of artwork, and one of the things I enjoy about looking at Jack's career as a whole is watching him continue to grow as an artist and storyteller. Jack is a great illustrator to study if you want to explore someone consciously working to create a more effective and innovative style on every single page day by day over a period of five decades.