The original North American area codes were established by the American Telephone and Telegraph Company (AT&T) in 1947. The assignment was in accord with the design of a uniform nationwide telephone numbering plan that supported the goal of dialing any telephone in the nation without involvement of operators at each routing step of a telephone call from origination location to its destination. The new technology had the aim of speeding the connecting times for long-distance calling by eliminating the intermediary telephone operators and reducing cost. It was initially designed and implemented for Operator Toll Dialing, in which operators at the origination point would dial the call as instructed by service subscribers, but had also the benefit of preparing the nation for Direct Distance Dialing (DDD) by customers years later. The nationwide and continental application followed the demonstration of regional Operator Toll Dialing in Philadelphia during the World War II period.

The new numbering plan established a uniform destination addressing and call routing system for all telephone networks in North America which had become an essential public service. The project mandated the conversion of all local telephone numbers in the system to consist of a three-character central office code and a four-digit station number.

The initial "Nationwide Numbering Plan" of 1947 established eighty-six numbering plan areas (NPAs) that principally conformed to existing U.S. state and Canadian provincial boundaries, but fifteen states and provinces were subdivided further. Forty NPAs were mapped to entire states or provinces. Each NPA was identified by a three-digit area code used as a prefix to each local telephone number. The United States received seventy-seven area codes, and Canada nine. The initial system of numbering plan areas and area codes was expanded rapidly during the ensuing decades, and established the North American Numbering Plan (NANP).

Early in the 20th century, the American and Canadian telephone industry had established criteria and circuits for sending telephone calls across the vast number of local telephone networks on the continent to permit users to call others in many remote places in both countries. By 1930, this resulted in the establishment of the General Toll Switching Plan, a systematic approach and network with technical specifications for routing calls between two major classes of routing centers, Regional Centers and Primary Outlets, as well as thousands of minor interchange points and tributaries. Calls were forwarded manually between stations by long-distance operators who used the method of ringdown to command remote operators to accept calls on behalf of customers. This required long call set-up times with several intermediate operators involved. For initiating a call, the originating party would typically have to hang up and be called back by an operator once the call was established.

The introduction of the first Western Electric No. 4 Crossbar Switching System in Philadelphia to commercial service, in August 1943, automated the process of forwarding telephone calls between regional switching points. For the Bell System this was the first test to let their long-distance operators dial calls directly to potentially distant telephones. While automatic switching decreased the connection times from as much as fifteen minutes to approximately two minutes for calls between far-away locations, each intermediate operator still had to determine special routing codes unique to their location for each call. To make a nationwide dialing network an efficient, practical reality, a uniform nationwide numbering plan was needed so that each telephone on the continent had a unique address that could be used independently from where a call originated. Such a methodology is called destination routing.

With this goal, AT&T developed a new framework during the early 1940s, termed Operator Toll Dialing, which was begun by the installation of a newly developed toll switching system in Philadelphia in 1943. In 1945, the American Telephone and Telegraph Company declared this effort a major post-war project for the Bell System, and proceeded with periodical communications to the general telecommunication industry via the Dial Interexchange Committee of the United States Independent Telephone Association (USITA), which disseminated the project's progress to its members via industry journals and conference contributions. The planning transitioned to implementation, when Ralph Mabbs presented the results in a talk at the Fiftieth Anniversary Meeting of the Independent Telephone Association, on October 14, 1947.

A fundamental requirement for the success of automated toll dialing was a new telephone numbering plan, which became known as the Nationwide Numbering Plan. This numbering plan accounted initially for seventy-seven area codes in the United States and nine in Canada. With the build-out and detailed analysis of existing technical infrastructure for toll dialing, the allocation maps needed to be modified in many states, adding numerous additional area codes during the next decade. By 1975, the numbering plan was known as the North American Numbering Plan, as efforts were in progress to expand the system beyond the United States and Canada.

Building a nationwide network in which any telephone could be dialed directly from anywhere in the country required a systematic numbering system that was easy to understand and communicate. Existing local telephone numbers varied greatly across the country, from two or three digits in very small communities, to seven in the large cities.