United Airlines operated 96 different DC-6 aircrafts between 1946 and 1968.
Registrations used were:

N37501, N37502, N37503, N37504, N37505, N37506, N37507, N37508, N37509, N37510
N37511, N37512, N37513, N37514, N37515, N37516, N37517, N37518, N37519, N37520
N37521, N37522, N37523, N37524, N37525, N37526, N37527, N37528, N37529, N37530
N37531, N37532, N37533, N37534, N37535, N37536, N37537, N37538, N37539, N37540
N37541, N37542, N37543, N37544, N37545, N37546, N37547, N37548, N37549, N37550
N37551, N37552, N37553, N37554, N37555, N37556, N37557, N37558, N37559, N37560
N37561, N37562, N37563, N37564, N37565, N37566, N37567, N37568, N37569, N37570
N37571, N37572, N37573, N37574, N37575, N37576, N37577, N37578, N37579, N37580
N37581, N37582, N37583, N37584, N37585, N37586, N37587, N37588, N37589, N37590
N37591, N37592, N37593, N37594, N37595, N37596.

In the collection 6 of the 96 aircrafts are represented.

***

N37503
Construction number 42868, model Douglas DC-6.
Delivered in November 1946 named Grand Coulee and given fleet number 5203.
In service until August 1968.

N37537
Construction number 43025, model Douglas DC-6.
Delivered in July 1948 named Texas. Later renamed to Nantucket and given fleet number 5237.
In service until December 1968.

N37555
Construction number 43276, model Douglas DC-6B.
Delivered in January 1952 named Mainliner New York and given fleet number 6555.
In service until October 1968.

N37559
Construction number 43538, model Douglas DC-6B.
Delivered in February 1952 named Mainliner Denver and given fleet number 6559.
In service until November 1st 1955.

From Wikipedia: United Airlines Flight 629, registration N37559, was a Douglas DC-6B aircraft, named "Mainliner Denver," which was blown up with a dynamite bomb placed in the checked luggage. The explosion occurred over Longmont, Colorado while the airplane was en route from Denver, Colorado to Portland, Oregon, and Seattle, Washington, on November 1, 1955. All 39 passengers and five crew members on board were killed in the explosion and crash. Investigators determined that Jack Gilbert Graham was responsible for bombing the airplane to kill his mother and obtain a large life insurance policy on her. Graham was tried, convicted and executed for his crime.

N37562
Construction number 43541, model Douglas DC-6B.
Delivered in April 1952 named Mainliner San Francisco and given fleet number 6562.
In service until September 1968.

N37581
Construction number 45133, model Douglas DC-6B.
Delivered in May 1957 named Mainliner Spokane and given fleet number 6581.
In service until 19??.

United Airlines operated 57 different DC-7 aircrafts between 1954 and 1964.
Registrations used were:

N6301C, N6302C, N6303C, N6304C, N6305C, N6306C, N6307C, N6308C, N6309C, N6310C
N6311C, N6312C, N6313C, N6314C, N6315C, N6316C, N6317C, N6318C, N6319C, N6320C
N6321C, N6322C, N6323C, N6324C, N6325C, N6326C, N6327C, N6328C, N6329C, N6330C
N6331C, N6332C, N6333C, N6334C, N6335C, N6336C, N6337C, N6338C, N6339C, N6340C
N6341C, N6342C, N6343C, N6344C, N6345C, N6346C, N6347C, N6348C, N6349C, N6350C
N6351C, N6352C, N6353C, N6354C, N6355C, N6356C, N6357C

In the collection 3 of the 57 aircrafts are represented.

***

N6301C
Construction number 44265, model Douglas DC-7.
Delivered in April 1954 named City of San Francisco and given fleet number 9101.
In service until 19??.

N6302C
Construction number 44266, model Douglas DC-7.
Delivered in April 1954 named Mainliner New York and given fleet number 9102.
In service until January 1964.

N6330C
Construction number 44266, model Douglas DC-7.
Delivered in January 1957. Named Mainliner Springfield.
In service until March 1964.

United Airlines operated 117 different DC-8 aircrafts between 1960 and 1984.
Registrations used were

N804PA
N8001U, N8002U, N8003U, N8004U, N8005U, N8006U, N8007U, N8008U, N8009U, N8010U
N8011U, N8012U, N8013U, N8014U, N8015U, N8016U, N8017U, N8018U, N8019U, N8020U
N8021U, N8022U, N8023U, N8024U, N8025U, N8026U, N8027U, N8028U, N8029U, N8030U
N8031U, N8032U, N8033U, N8034U, N8035U, N8036U, N8037U, N8038U, N8039U, N8040U
N8041U, N8042U, N8043U, N8044U, N8045U, N8046U, N8047U, N8048U, N8049U, N8050U
N8051U, N8052U, N8053U, N8054U, N8055U
N8060U
N8061U, N8062U, N8063U, N8064U, N8065U, N8066U, N8067U, N8068U, N8069U, N8070U
N8071U, N8072U, N8073U, N8074U, N8075U, N8076U, N8077U, N8078U, N8079U, N8080U
N8081U, N8082U, N8083U, N8084U, N8085U, N8086U, N8087U, N8088U, N8089U, N8090U
N8091U, N8092U, N8093U, N8094U, N8095U, N8096U, N8097U, N8098U, N8099U
N8177U, N8207U, N8209U, N8215U, N8243U, N8245U, N8246U, N8252U, N8258U, N8266U
N8966U, N8967U, N8968U, N8969U, N8970U, N8971U, N8972U, N8973U, N8974U, N8975U

In the collection 1 of the 117 aircrafts are represented.

***

N8028U
Construction number 45297, model Douglas DC-8-21.
Delivered in June 1960 named Mainliner Waipahu and given fleet number 2728.
In service until 1978.

United Airlines operated 7 different Boeing 377 Stratocruiser aircrafts between 1949 and 1954.
One aircraft crashed 1951 (N31230), the other 6 were sold to BOAC in 1954.
Registrations used were:

N31225, N31226, N31227, N31228, N31229, N31230, N31231

In the collection 2 of the 7 aircrafts are represented.

***

N31225
Construction number 15965, model Boeing 377-10-34.
Delivered in September 1949 named Mainliner Hawaii.
In service until 1954 when sold to BOAC. New registration became G-ANTX.

N31226
Construction number 15966, model Boeing 377-10-34.
Delivered in October 1949 named Mainliner Kauai.
In service until 1954 when sold to BOAC. New registration became G-ANTY.

Unidentified registrations

Interior views

United Airlines operated 237 different Boeing 727 aircrafts between 1963 and 2001.
Registrations used were N7001U-N7050U, N7052U-N7090U, N7251U-N7299U, N7401U-N7438U, N7441U-N7467U, N7620U-N7647U, N693WA-N695WA, N530EJ-N531EJ

In the collection 0 of the 237 aircrafts are depicted.

United Airlines operated 20 different Caravelle aircrafts between 1961 and 1972.
Registrations used were N1001U-N1020U

In the collection 1 of the 20 aircrafts are depicted.