"The Case of the Howling Dog," made in 1934, was the first Perry Mason film, and it's from an actual Erle Stanley Gardner Perry Mason novel. One way you can tell is that it's not an easy plot to follow. Mason becomes embroiled in defending a woman (Mary Astor) for the murder of her husband. Meanwhile, her husband and the dead man's wife are missing. And then there's that howling dog.
A really excellent story, but Erle Stanley Gardner loathed what the movies did to his passionate young Depression lawyer. Perry here has a huge office and is too big to take certain cases; Della is there, but not Paul Drake or even Gertie the switchboard operator. Warren William is a clever, serious Perry, and gives the impression of a lawyer to be reckoned with. He also has occasion touches of humor, though if memory serves, there's a lot more humor in the later films.
As one who read the original Perry Mason books, the character matures and becomes less given to speeches about the law - William would perhaps have been better as the later Perry, though Gardner himself never would have chosen him. He wanted Fred MacMurray until Raymond Burr walked in to read for the role of the D.A. He then said, "That's Perry Mason." Despite some of the stilted dialogue, this is still a very good story and well worth seeing. Mary Astor is lovely as the defendant.
As one of the comments pointed out, the very talented Lightning the Dog is uncredited, but to say more would give away the plot. Let's just say Lightning is a fine actor and leave it at that.