M31 may refer to:

Roadways

M31 motorway The M31 was planned as a Reading to M3 motorway which was dubbed the 'M3 – M4 link motorway'. It would have provided a direct high-speed route between the two motorways. The motorway was planned at the same time as the largely unrealised Lo ...

, a planned but unbuilt motorway in England *

M-31 (Michigan highway) M-31 was a state trunkline highway in the Lower Peninsula in the US state of Michigan. It generally ran north from Port Huron along the Lake Huron shoreline through The Thumb region before turning inland. The highway crossed The Thumb and then ...

, a former state highway in Michigan *

Hume Highway Hume Highway, inclusive of the sections now known as Hume Freeway and Hume Motorway, is one of Australia's major inter-city national highways, running for between Melbourne in the southwest and Sydney in the northeast. Upgrading of the route ...

in Australia, designated "M31" *

M31 motorway (Hungary) The M31 motorway ( hu, M31-es autópálya) is a motorway in Hungary, that runs between the M0 motorway at Nagytarcsa and the M3 motorway at Gödöllő. Municipalities The M31 motorway runs through the following municipalities: * Pest County: B ...

, a short motorway section near Budapest, Hungary * M31 (Cape Town), a Metropolitan Route in Cape Town, South Africa *

M31 (Johannesburg) The M31 is a metropolitan route in Johannesburg, South Africa. The road connects the southern suburbs of Alberton with Johannesburg's northern suburbs. The routes eastern bypass in the Johnannesburg CBD connects the M2 motorway at the Heidelber ...

, a Metropolitan Route in Johannesburg, South Africa *

M31 (Pretoria) The M31 road is a metropolitan route in the City of Tshwane in Gauteng, South Africa. It connects the southern suburbs of Centurion, from the Rietvlei Nature Reserve to Rooihuiskraal via Irene and Highveld. Route The M31 route begins just ...

, a Metropolitan Route in Pretoria, South Africa *

M31 (Durban) Ethekwini (Durban metropolitan area), like most South African metropolitan areas, uses Metropolitan or "M" routes for important intra-city routes, a layer below National (N) roads and Regional (R) roads. Each city's M roads are independently n ...

, a Metropolitan Route in Durban, South Africa

Military

Suomi M-31 SMG The Suomi KP/-31 ( or "Finland-submachine gun mod. 1931") is a submachine gun (SMG) of Finnish design that was mainly used during World War II. It is a descendant of the M-22 prototype and the KP/-26 production model, which was revealed to the p ...

, a Finnish WWII-era submachine gun *

M31 HEAT rifle grenade The M31 HEAT is a fin-stabilized anti-tank rifle grenade designed in the late 1950s to replace the Belgian ENERGA rifle grenade which was adopted by the US Army and US Marines as an emergency stop-gap measure during the Korean War. Like the EN ...

* M31 Recovery Vehicle based on the

M3 Lee The M3 Lee, officially Medium Tank, M3, was an American medium tank used during World War II. The turret was produced in two forms, one for US needs and one modified to British requirements to place the radio next to the commander. In British Co ...

tank * M31 rocket, an alternate designation for the

MGR-1 Honest John The MGR-1 Honest John rocket was the first nuclear-capable surface-to-surface rocket in the United States arsenal.The first nuclear-authorized ''guided'' missile was the MGM-5 Corporal. Originally designated Artillery Rocket XM31, the first un ...

, a United States military rocket *

M31 (missile) The M270 Multiple Launch Rocket System (M270 MLRS) is an American-developed armored, self-propelled, multiple rocket launcher. The U.S. Army variant of the MLRS vehicle is based on the chassis of the Bradley Fighting Vehicle. The first M270s ...

, for a multiple-launch rocket system (MLRS) * M31 is the pennant number for HMS ''Cattistock'', a Hunt-class mine countermeasures vessel

Other

* M₃₁ = 0x7FFFFFFF =

2147483647 The number 2,147,483,647 is the eighth Mersenne prime, equal to 231 − 1. It is one of only four known double Mersenne primes. The primality of this number was proven by Leonhard Euler, who reported the proof in a letter to Daniel ...

, the eighth Mersenne prime number *

M31 (New York City bus) Several companies, most prominently the Metropolitan Transportation Authority (MTA), operate a number of bus routes in Manhattan, New York, United States. Many of them are the direct descendants of streetcar lines (see list of streetcar lines in ...

, a New York City Bus route in Manhattan * ''M31: A Family Romance'', a novel by writer Stephen Wright *

Messier 31 Messier may refer to: People with the surname *Charles Messier, French astronomer *Éric Messier, former NHL defenseman *George Messier, French inventor *Jean-Marie Messier, former CEO of Vivendi Universal *Marc Messier, Canadian actor from Quebec ...

(M31), a spiral galaxy also called the Andromeda Galaxy * Samsung Galaxy M31, an Android based smartphone {{Letter-Number Combination Disambiguation