Where does "Let's roll!" come from?

Solution 1:

The phrase "Let's roll" has been in use as early as 1908, but achieved a special connotation on September 11th, 2001, as they were Todd Beamer's final words before rushing the cockpit on United Airlines Flight 93.

Wikipedia cites a possible origin of the word from 1908:

The phrase may have its origins as early as 1908 in the cadence song now called "The Army Goes Rolling Along", which likely extended into tank usage.

The same Wikipedia page also provides several other uses of the phrase in pop culture, including the 1950 Hemingway novel Across the River and into the Trees and its use in 50s and 60s police television shows such as Adam-12 and (the original) Dragnet.

Solution 2:

This Glossary of Colloquialisms mentions "Let's roll" as:

1. A term to move and start an activity, attack, mission or project.
2. A symbol of heroism and initiative in danger.

Example:

The phone line from Flight 93 was still open when a GTE operator heard Todd Beamer say: "Are you guys ready? Let's roll!".

It defines its history as follow:

"Let's roll" was in common use on 1950s and 1960s police television shows such as "Adam-12" and (the original) "Dragnet".

But its usage on 9/11 changed its meaning.

Todd Beamer, a passenger on the hijacked United Airlines Flight 93, called his wife on the plane's seat back GTE airfone after the flight was hijacked. Through that phone and other phone contacts with the ground, the passengers learned that two other hijacked planes had been crashed into the World Trade Center.
As a result, some of the passengers apparently decided to storm the cockpit. Beamer spoke his last known words to the group, overheard via the phone connection: "Let's roll".

The catchphrase became especially known and popular after being used by President George W. Bush in a speech to AmeriCorps volunteers and during his 2002 State of the Union Address. Profiteers soon tried to lay claim to it as a trademark, even though the phrase was in common use long before September 11.

In early 2002, United States Air Force Chief of Staff Gen. John P. Jumper ordered that one airplane in each USAF squadron and all USAF demonstration planes would bear an image of an eagle on an American flag with the words "Let's Roll" and "Spirit of 9-11", to remain until the first anniversary of the attack.

The phrase was also used as the title of a Neil Young song about the flight. It was also used by Lisa Beamer, widow of Todd, in a 2003 book titled "Let's Roll: Ordinary People, Extraordinary Courage".