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Military aircraft exist in a complicated world. They are built to serve a particular mission set, but that mission set might not exist by the time they are developed, or the way they are designed to fill it may have become obsolete. Sometimes, even commercial passenger jet projects face the prospect of collapse before entering service.
For example, Canada’s Bombardier developed the CSeries regional airliner, but the cost of the project almost brought the company down. It was saved by a massive order from 
Delta Air Lines, but even so, Bombardier was forced to sell it to Airbus as the Airbus A220 and exit the commercial passenger aircraft business. Here is what to know about some aircraft that flew well, but failed to enter serial production.
Avro Canada CF-105 Arrow
5 prototypes built
During World War II, Canada built one of the world’s largest aerospace sectors, producing many of Britain’s military aircraft. Canada built over 16,000 aircraft of various descriptions, including Avro Lancasters, Hawker Hurricanes, and de Havilland Mosquitos. Canada had grown its aerospace base from eight small factories producing 40 aircraft per year to the fourth largest in the world. Post-war, Avro Canada designed the CF-100 Canuck, Canada’s first jet fighter. The type first flew in 1950 and entered service in 1952 with a total of 692 examples built.
Avro Canada also began work on a jet-powered civilian short-to-medium-range commercial aircraft called the C102 Jetliner, but the aircraft ran into problems after the UK blocked the export of its Rolls-Royce Avon AJ-65 engines. Avro Canada turned to its most famous effort, the CF-105 Arrow interceptor. The aircraft was first rolled out in October 1957 and is remembered in Canada for breaking many records and setting many firsts.
The first flight took place in March 1958. But the Canadian government controversially canceled the program in 1959. The decision was disastrous for Canadian industry, which not only saw the tooling destroyed, but also saw many of the engineers leave Canada for the US and UK. The Arrow continues to conjure up both national pride and sorrow in Canada today.
North American XB-70 Valkyrie
2 prototypes built
The B-52 Stratofortress can be seen as the last of a line of WWII bombers (B-17, B-29, B-36) that were built to fly en masse and brute force their way through enemy airspace. With these bombers, losses and attrition were expected. The Air Force then looked for a bomber that could outrun jet fighter interceptors. The US Air Force built the B-58 Hustler that could fly at Mach 2.0 and at 50,000 feet, making it difficult for enemy aircraft to intercept.
The Air Force took this concept to the next level and developed the massive North American XB-70 Valkyrie. The Valkyrie was to fly above enemy air defense at around 77,000 feet and outrun any jet fighters at a maximum Mach 3.1. However, as the US built the XB-70, the Soviets developed surface-to-air missiles that effectively made the XB-70 obsolete overnight.
Only two experimental Valkyries were built and flown. In 1966, one of the two aircraft (XB-70A No. 2) was destroyed after a midair collision with an F-104. The surviving airframe was retired in 1969 and is now preserved at the National Museum of the United States Air Force.
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Northrop YF-23
2 prototypes built
The Northrop YF-23 differs from the aircraft above in that it was one of two competing demonstrator aircraft, the other being the Lockheed Martin YF-22, which would go on to be the Lockheed Martin F-22 Raptor. The F-22 competition is remembered because both Northrop and Lockheed presented excellent designs, although the Air Force had to choose one.
Even today, many enthusiasts claim the competition was stolen from Northrop, and the YF-23 should rightfully have been the winning design. The YF-23 did outperform the YF-22 in some substantial ways, including being considered stealthier, faster, and having a longer unrefueled range. It was even able to supercruise at higher speeds.
The two demonstrators were about equal when it came to thrust-to-weight ratios and payloads. However, the YF-22 was a more developed demonstrator, able to win the presentation battle, perform more aerobatic maneuvers, and impress professionals. Another factor was that, at the time, Lockheed was in much better shape, while Northrop was beset by scandals and had pleaded guilty to 34 fraud charges. For a range of reasons, the YF-23 was sidelined in favor of the YF-22.
Eclipse 400
1 prototype built
Not all aircraft that flew brilliantly but failed to make it into service are military. One example of an eye-catching civil utility aircraft that almost made it is the Eclipse 400 (previously called the Eclipse Concept Jet, or ECJ). The aircraft was a single-engine very light jet developed from the twin-engine very light Eclipse 500.
The Eclipse 400 was unveiled in 2007 and was intended to compete with other single-engined light aircraft like the Cirrus Vision SF50, the Piper PA-47 PiperJet, the Epic Victory, and the Diamond D-Jet. The Eclipse was powered by a Pratt & Whitney Canada PW615F turbofan and the prototype aircraft only came with four seats.
The Eclipse 400 was canceled after Eclipse Aviation fell into bankruptcy in 2008 and became defunct in 2009. Eclipse built 260 examples of its Eclipse 500 aircraft before production ended with bankruptcy. Notably, only one Piper PA-47 PiperJet was built before that competing very light jet program was also canceled in 2008.
Martin XB-51
2 prototypes built
The Martin XB-51 is regarded as one of the most unusual, but advanced, jet attack aircraft built in the early Cold War. The XB-51 flew, but never entered production and remains one of the great “what could have beens.” The XB-51 was built by Glenn L. Martin Company and first flew in 1949 before being canceled in 1952.
The aircraft had a speed of 645 mph (high for the time) and wings that could change angle, allowing for shorter takeoff runs and improved low-speed handling. It was a trijet ground-attack aircraft that remains remembered for its unorthodox design. Only two flying prototypes were built, both of which crashed.
The aircraft lost the competition to the British-designed English Electric Canberra, which came with a much longer range and longer endurance. Canberras were also simpler and more practical. Even so, in a twist of fate, Martin ended up getting the license to build the Martin B-57 Canberra for the Air Force. Three modified WB-57 Canberras remain in service with NASA, and one of them provided the high-altitude footage of the launch of Artemis II in 2026.
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Douglas DC-1
1 example was built
The Douglas DC-1 is listed here as an example of how an aircraft can fly so brilliantly that it never enters production. Douglas developed the DC-1 in 1933 to meet the requirements of TWA for an aircraft to compete with the Boeing Model 247. Donald Douglas designed the aircraft as a low-wing, twin-engined, all-metal aircraft able to seat 12 passengers (later increased to 14 passengers) plus three crew.
Douglas had doubts about the wisdom of designing the aircraft, as he doubted there was a market for 100 of them, the number needed to break even. The DC-1 immediately proved superior to all other competing aircraft, like the Ford Trimotor and Fokker Trimotor. The aircraft was so good that TWA wanted more, but it was modified into the larger Douglas DC-2. Only one DC-2 was ever built.
A total of 192 DC-2s were built and flew with TWA, KLM, and Pan Am. It was modified into the larger DC-3, which became one of the most successful aircraft of all time. Some 607 commercial passenger DC-3s were built, with the aircraft also developed into the Douglas C-47 Skytrain and other military variants. Over 10,000 C-47s were built, and these had a massive impact on maturing the still fledgling commercial passenger industry after WWII.
Yakovlev MC-21-300
7 prototypes built (all variants)
The MC-21 is listed here as a sort of gray zone aircraft. Russia developed the narrowbody MC-21, and it was set to enter serial production before the West slammed sanctions on Russia in 2022. This was crippling for the aircraft as over half of its components, including its engines, were imported from the West. Since then, Russia has been Russifying the aircraft by substituting imported components and systems with its own domestic products.
But this has essentially created a new inferior aircraft. The MC-21-300 was powered by PW1400G geared turbofans, while the Russified MC-21-310 version is powered by Aviadvigatel PD-14 engines. Almost all of its systems have been replaced. The resulting aircraft is around five to six tons heavier, has a 25% shorter range, has a reduced payload, has a higher fuel burn, and has higher operating costs.
From one point of view, the MC-21 has survived sanctions and, having moved to serial production this year, is poised to enter commercial service in early 2027. From the perspective of airlines, this aircraft is not what they ordered and is essentially a different aircraft. The airframe may be the same, but the systems, software, and underlying hardware are all different. The MC-21 was meant to compete with the Boeing 737 MAX, but the aircraft that will actually enter service is closer to the Boeing 737 NG or Classic in terms of efficiency.
