Earliest ones were not, later ones were. Howard, like Fred Bear, was always looking for ways to improve durability and performance of their equipment. Here is a quote from Bob Burton's excellent book "Howard Hill Collectibles":
"Fiberglassing began in the late 1940's. In the 1950's Howard was at the peak of his popularity, going to Africa that year. Being a business man he knew his time was more valuable on stage performing or doing movies. I'm not saying he didn't make any laminated glass bows. What I am saying is that those he did make would have been for himself or close personal friends, probably not for the general public. In the following pages let's look at some bows that were made by Howard without glass from the mid 1920's through the early 1950's. You'll notice a great deal of change during those 30 years. He was one that was continuously experimenting. You will find bows backed with just about anything. He used various kinds of fiber material, rawhide, tanned leather, and sinew." Page 13.