20th century

Planning for a new airport on 3,200 acres (1,300 ha) to serve the Baltimore–Washington metropolitan area began in 1944, just prior to the end of World War II, when the Baltimore Aviation Commission announced its decision that the best location to build a new airport would be on a 2,100-acre (850 ha) tract of land near Linthicum Heights, Maryland. The cost of building the airport was estimated at $9 million.

The site was chosen because it was a 15-minute drive from Downtown Baltimore, close to the Pennsylvania Railroad line, the Baltimore and Annapolis Railroad line, and the proposed Baltimore–Washington Parkway, and visibility at the site was generally good. An alternate site along Gov. Ritchie Highway at Furnace Branch was rejected by the United States Department of War, and another possible site at Lipin's Corner was deemed too far from Baltimore. The State Aviation Commission approved of the Linthicum Heights site in 1946.

Much of the land was purchased from Friendship Methodist Church in 1946, and ground was broken on May 2, 1947. Friendship Methodist Church held its last service on Easter Sunday in 1948. Friendship Methodist Church was razed to make room for the new airport. In addition, several pieces of land were bought, and 170 bodies buried in a cemetery were moved. Baltimore–Fort Meade Road was moved to the west to make way for the airport's construction.

Friendship International Airport was dedicated on June 24, 1950, by President Harry S. Truman. Truman arrived in a Douglas DC-6, then the official presidential airplane, from nearby Washington National Airport. Accompanying Truman were the Governor William Preston Lane Jr. and Baltimore Mayor Thomas D'Alesandro Jr., who was taking his first aircraft flight. The cost to construct the airport totaled $15 million. The following month, the airlines moved to the new airport from the old Baltimore Municipal Airport at Harbor Field in southeast Baltimore at 39.25°N 76.53°W / 39.25; -76.53. Eastern Airlines flew the first scheduled flight, a DC-3, into the airport at 12:01 am on July 23, 1950. Seven minutes later, the same plane was also the first flight to depart from the airport. Three hundred spectators came to watch the first flights arrive and depart.

The Official Airline Guide reports 52 weekday departures from the airport as of April 1957: 19 Eastern, 12 Capital, 8 American, four National, three TWA, three United, two Delta, and one Allegheny. The departures included a couple nonstop flights to Miami, but westward nonstop flights did not reach beyond Ohio. The airport's reach expanded when jet service started. The early Boeing 707s and Douglas DC-8s could not use Washington National Airport, and Dulles International Airport, which opened in 1962, had not yet opened, so Baltimore became Washington, D.C.'s jet airport from May 1959 to June 1959, when American and TWA began transcontinental 707 flights. By 1963, Friendship Int'l Airport was equipped with a 9,450 feet (2,880 m) runway, which could handle any commercial jet aircraft at that time.

In 1972, the Maryland Department of Transportation (MDOT) purchased Friendship International Airport from the City of Baltimore for $36 million. Under MDOT, the Maryland State Aviation Administration took over airfield operations, and the airport grew from three employees to more than 200. Plans to upgrade, improve, and modernize all Maryland airport facilities were announced almost immediately by Harry Hughes, then Maryland Secretary of Transportation and later Governor of Maryland.

On November 16, 1973, in an effort to attract passengers from the Washington metropolitan area, particularly Montgomery and Prince George's counties in suburban Maryland, the airport was renamed Baltimore/Washington International Airport. Its IATA code, originally BAL, was changed to BWI by the International Air Transport Association on April 20, 1980, and the change became official six months later, on October 26. The BWI code had previously been used by an airport in Bewani, Papua New Guinea.

In 1974, the first phase of the airport's modernization was completed at a cost of $30 million. Upgrades included improved instrument landing capabilities and runway systems, and construction of three new air cargo terminals, expanding the airport's freight capacity to 2.53 acres (1.02 ha).

In 1979, the terminal renovation program was completed, representing the most dramatic work of the airport's modernization, which was designed by DMJM along with Peterson & Brickbauer. The renovations more than doubled the size of BWI's terminal to 14.58 acres (5.90 ha), and the number of gates increased from 20 to 27. The total cost was $70 million. To continue the work, the BWI Development Council was established to support initiatives for airport development.

In 1980, the BWI Rail Station opened, providing a connection for passengers on the Northeast Corridor through Amtrak. BWI was the first airport in the U.S. with a dedicated intercity rail station.