The six pioneering women who programmed the world's first electronic computer
In 1945, the United States unveiled ENIAC, the first general-purpose, fully electronic computer. While male engineers designed the hardware, six brilliant women—Kay McNulty, Betty Jennings, Betty Snyder, Marlyn Wescoff, Frances Bilas, and Ruth Lichterman—were tasked with programming it. They had to configure the machine physically using thousands of switches and patch cables, establishing the foundational concepts of modern software development.
From Human Calculators to Machine Operators
During World War II, the United States military faced a critical mathematical bottleneck: producing artillery firing tables. These tables allowed gunners in the field to adjust their aim based on variables such as distance, wind speed, elevation, and atmospheric pressure. Calculating a single trajectory required solving dozens of complex differential equations, a task carried out by teams of human mathematical workers known simply as "computers." The vast majority of these computers were women with backgrounds in mathematics.
The calculations were intensely labor-intensive, often taking several days or weeks per firing table when done by hand using mechanical desk calculators. To break this logjam, the U.S. Army funded the construction of the Electronic Numerical Integrator and Computer (ENIAC) at the University of Pennsylvania's Moore School of Electrical Engineering. As chief engineers J. Presper Eckert and John Mauchly developed the hardware, six human computers were selected to run calculations on the new machine: Kathleen McNulty, Frances Bilas, Betty Jean Jennings, Ruth Lichterman, Elizabeth Snyder, and Marlyn Wescoff.
Wiring the Logic: How Early Programming Worked
When the six women began their assignment, modern programming languages, operating systems, and software compilers did not exist. ENIAC was not originally a stored-program computer; it could not read instructions sequentially from internal memory. Instead, its mathematical logic had to be configured directly into the hardware using patch cords, rotary switches, and plugboards across its massive U-shaped array of panels.
To make the machine execute a problem, the programmers had to understand ENIAC from the inside out. They were given block diagrams and circuit schematics rather than user manuals. From these electrical blueprints, they deduced how the machine's individual units—including twenty accumulators, a multiplier, a divider and square-rooter, and function tables—handled data pulses. Programming meant designing a parallel flow of electrical signals across these units, then physically stepping up to the machine to plug in heavy cables and set hundreds of switches to route those signals correctly.
Inventing Foundational Principles of Software
Translating a mathematical equation into physical machine states required creating methods that later became foundational to computer science. The programmers broke down complex differential equations into discrete, step-by-step arithmetic operations. Because ENIAC could execute operations in parallel across different panels, the women had to synchronize the timing of pulses so that intermediate values arrived at the proper unit at precisely the right moment.
Through this work, they pioneered practical concepts of control flow, including loops, nested subroutines, and conditional branching. When a run failed, they had to determine whether the fault lay in a logic error in their routing design or a physical hardware breakdown, such as a burned-out vacuum tube among the machine's nearly 18,000 tubes. This process established the earliest practices of software debugging.
Oversight, Erasure, and Recognition
When ENIAC was officially unveiled to the public and press in early 1946, it successfully calculated a ballistic trajectory in seconds—faster than the physical flight of the shell itself. The demonstration was a major milestone in technology, but the public narrative focused almost entirely on the male engineers who designed the physical structure. In press photographs, the women seen adjusting the switches were often uncredited or assumed to be clerical assistants and demonstrator models.
Several of the programmers continued to shape early computing after ENIAC. Betty Holberton (formerly Snyder) helped develop sorting algorithms and early programming language standards, while Jean Bartik (formerly Jennings) worked on the logic for converting ENIAC into a stored-program computer and contributed to later systems like BINAC and UNIVAC. Over subsequent decades, historians recovered and documented their central role, establishing that the discipline of software engineering emerged directly from their hands-on work with the first electronic computer.
Key takeaways
•ENIAC was programmed physically by routing cables and setting switches across modular hardware units rather than entering digital code.
•The six programmers had no programming languages or manuals, developing control flow, subroutines, and debugging by analyzing electrical schematics.
•Though initially uncredited in wartime and post-war publicity, their work established the foundational practices of modern software development.