Daring Mystery Comics #4 (May 1940)
Some sources, including the GCD, attribute the feature Trojak from Daring Mystery #4 to Joe Simon. Joe had started producing covers for Fox with cover dates of May. However that would not necessarily indicate that Simon was unavailable to produce stories for Timely. Joe would create Blue Bolt which started in June so he did not work exclusively for Fox.
This particular story is not signed, that by itself is rather unusual for Joe. Before starting for Fox Joe seemed to have signed all his comic book work, including covers although he sometime used an alias. At Fox Joe did not sign all the covers, still 9 out of 16 Fox covers that he did are signed. Joe signed all stories that he drew until he teamed up with Jack Kirby. After that not every thing is signed and if there is a signature it is the joint Simon and Kirby signature. Even the Fiery Mask from Human Torch #1 (Fall 1940) which was drawn by Joe alone, is not signed. So an unsigned story by Joe published in May would be unique.
Simon had his own personal drawing style, but he mimicked other artists at times. For the Fox covers Joe copied Lou Fine’s manner since Lou had done a number of Fox covers previously. When Joe teamed up with Jack he would begin to adopt Kirby’s manner. But this Trojak story was before the Simon and Kirby team up. Further Joe was the original creator for this feature. So there would seem to be no reason for Simon to mimic any other artist. Therefore we should expect Joe’s own unique style. But that style is not found here. The depiction of the woman is the closest to Joe’s but even she can be distinguished. Simon had a technique of joining eyebrows and eyes into a single angular formation, but that also does not show up in this story. I am not sure who the artist was, but he was not Joe.