Staff Writer (Updated: 7/18/2016):
Ed Heinemann was the chief designer of the A-4 Skyhawk while working at Douglas, which led to the development of nine Skyhawk prototypes designated as the XA4D-1 - the first of which flew on September in 1956. From there, the Pratt & Whitney J65-powered craft would go into full operational production as the A-4A Skyhawk.
The Pratt & Whitney powerplant was a British-designed and licensed Armstrong Siddeley Sapphire turbojet. The system was capable of producing over 8,000lbs or standard thrust (no afterburner was integrated to the engine) and the base powerplant could be found on the initial three A-4's - the A, B and C models.
The A-4E Skyhawk model stood as a vast improvement over previous models and served the US Navy featuring an overall heavier airframe but a Pratt & Whitney J52 powerplant. A successive model for the US Navy, the A-4F, proved to be the last model and featured further improvements on the A-4E, most notably the avionics-housing "hump" visible in the image above just behind the cockpit and running along the dorsal spine of the fuselage.
The United States Marine Corps took orders for the "Ultimate Skyhawk" in the A-4M Skyhawk II. This model featured an all-new enlarged canopy for improved pilot visibility. It also featured an increased MTOW capacity (nearly double of what the base A-4 could do) allowing for an additional array of weaponry to be fitted. The A-4M model was fitted with an even more powerful J52-P-408 powerplant that increase range and overall speed.
Export figures were decent, with Argentina becoming the initial customer. Others would soon follow including Israel, Malaysia, Kuwait, Brazil and Singapore. In the end, the A-4 Skyhawk proved its worth with decades of faithful service to the United States with the Navy and the Marine Corps both putting the aircraft through a rigorous pacing. The single powerplant was more than suitable for the role that the A-4 was designed to fulfill and the diminutive size when compared to other carrier aircraft was a good fit for carrier storage.