Apple may have considered including USB hard drive support in the AirPort Express initially released in June of 2012, according to an early support manual unearthed by French site Journal Du Lapin (via iPhone in Canada).
Available in PDF format, the older manual references a section suggesting a USB hard disk or USB hub could be connected to the AirPort Express, allowing all computers on the network to access those devices.
Connect a self-powered USB hard disk to your AirPort Express. All of the computers on the AirPort network can access the information on the hard disk.
Connect a USB hub to your AirPort Express, and then connect multiple USB devices, such as printers or hard disks. All of the computers on the AirPort network can access those devices.
These references are not found in the final product's manual [PDF], which only includes a section on connecting a USB printer to the device. USB hard disk compatibility would have potentially allowed the lower-cost AirPort Express to function in the same way as the more expensive AirPort Extreme, suggesting Apple possibly once considered merging these two products before making the decision to release the AirPort Express without external hard drive compatibility.
While Apple's AirPort Extreme retails for $199 and includes the ability to connect to a USB hard drive that can be accessed by all computers on the network for sharing and storing files, the AirPort Express, priced $100 less at $99, remains a no-frills entry level WiFi solution for consumers for the time being.