Question: Which air base was at the western end of an American twin-engine, medium bomber's air route.

Context paragraphs:
Handley Page Hampden: The Handley Page HP.52 Hampden was a British twin-engine medium bomber of the Royal Air Force (RAF).  It was one of a trio of then-large twin-engine bombers procured for the RAF, the other two being the Armstrong Whitworth Whitley and Vickers Wellington.  The newest of the three medium bombers, the Hampden, was often referred to by aircrews as the "Flying Suitcase" because of its cramped crew conditions.

Mitsubishi Ki-67: The Mitsubishi Ki-67 "Hiryū" (飛龍, "Flying Dragon"; Allied reporting name "Peggy") was a twin-engine medium bomber produced by Mitsubishi and used by the Imperial Japanese Army Air Force and Imperial Japanese Navy Air Service in World War II.  Its Army long designation was "Army Type 4 Heavy Bomber" (四式重爆撃機).  Its Navy designations were "P2M" and "Q2M".

Columbia Army Air Base: Columbia Army Air Base was a World War II United States Army Air Forces base.  It was primarily used for advanced combat training of B-25 Mitchell medium bomber units and replacement pilots.

Cairo–Baghdad air route: The Cairo–Baghdad Air Route was an airmail route established by the Royal Air Force following a conference of British military and civil officials held in Cairo in March 1921.  The aim was to create an air link between Egypt, Mandate Palestine and British Mandate of Mesopotamia (Iraq), which were under British control following the end of World War I.  The western end of the route was the airfield at Heliopolis, on the outskirts of Cairo.  The eastern end was at Hinaidi airfield, just south of Baghdad.  It was intended the route would eventually extend to India.

Lioré et Olivier LeO 45: Lioré-et-Olivier LeO 45 was a French medium bomber that was used during and after the Second World War.  It had been originally designed and developed for the newly formed Armée de l'air as a modern medium bomber capable of performing independent strategic operations, unlike the majority of previous French bombers.

North American B-25 Mitchell: The North American B-25 Mitchell is an American twin-engine, medium bomber manufactured by North American Aviation (NAA).

Krasnoyarsk Northeast: Krasnoyarsk Northeast was an air base in Russia located 4 km northeast of Krasnoyarsk.  The airfield was the western end of the ALSIB Alaska-Siberia air route for Lend-Lease aircraft during World War II including Bell P-39 Airacobras and North American B-25 Mitchells.  The former airfield has been converted into apartment complexes.  There was an Antonov An-2 maintenance facility, which is now gone.  It was a utilitarian airfield with An-2, An-24, and An-6 aircraft, probably now based at Krasnoyarsk Yemelyanovo Airport.

Next-Generation Bomber: The Next-Generation Bomber (NGB; unofficially called 2018 Bomber) was a program to develop a new medium bomber for the United States Air Force.  The NGB was initially projected to enter service around 2018 as a stealthy, subsonic, medium-range, medium payload bomber to supplement and possibly—to a limited degree—replace the U.S. Air Force's aging bomber fleet (B-52 Stratofortress and B-1 Lancer).  The NGB program was superseded by the Long Range Strike Bomber (LRS-B) heavy bomber program.

Savoia-Marchetti SM.79: The Savoia-Marchetti SM.79 "Sparviero" (Italian for sparrowhawk) was a three-engined Italian medium bomber with a wood-and-metal structure.  Originally designed as a fast passenger aircraft, between 1937 and 1939 this low-wing monoplane set 26 world records, qualifying it for some time as the fastest medium bomber in the world.  It first saw action during the Spanish Civil War and flew on all fronts in which Italy was involved during World War II.  It achieved success as a torpedo bomber in the Mediterranean theater, and became the best-known Italian aeroplane of the war.  It was easily recognizable due to its distinctive fuselage dorsal "hump", and was well liked by its crews, who nicknamed it "il gobbo maledetto" ("damned hunchback").

Osan Air Base: Osan Air Base (K-55; Korean: 오산공군기지 ; Hanja: 烏山空軍基地 ), is a United States Air Force base located near Songtan Station in the city of Pyeongtaek, South Korea, 64 km south of Seoul.  Despite its name, Osan AB is not within Osan City, which is 7.5 km to the north.  The base is the home of the Pacific Air Forces' 51st Fighter Wing, and a number of tenant units, including the headquarters for Seventh Air Force.  The base is also the headquarters of the ROK Air Force Operations Command.  Osan Air Base is also the departure and arrival point for U.S. government-contracted "Patriot Express" flights bringing service members and their family members to South Korea from Seattle-Tacoma International Airport in the U.S. state of Washington, Misawa Air Base and Yokota Air Base in Japan.

Supporting paragraphs: Krasnoyarsk Northeast North American B-25 Mitchell
Answer: Krasnoyarsk Northeast