They originated in the Liverpool pub scene of the late fifties as the Quarrymen. Lennon and McCartney founded the band with George Harrison. The original lineup was five musicians including Stu Sutcliffe on bass and Pete Best on drums. Sutcliffe later dropped out causing McCartney to move to bass. The band then replaced Best with drummer Ringo Starr and the legendary Fab Four was complete.
The Beatles were largely influenced by fifties rock and skiffle music. They owed a lot, however, to the early rockabilly stars in the United States. In particular, they tend to cite Buddy Holly and Carl Perkins as key influences. In the Beatles earliest pop hits, the lyrical and melodic influences of Holly are clear. As for Carl Perkins, his influence on guitar is practically note-for-note in early Beatles guitar breaks.
Main Creative Force
Lennon and McCartney were both songwriters and both deserve a lot of credit for the Beatles' most successful hits. The later more progressive work that began to emerge on 'Sgt. Pepper's Lonely Hearts Club Band' and the White Album as well as 'Abbey Road' was still clearly more collaborative than one individual taking charge. All the songs on Sgt. Pepper's are credited to both men and songs like Lennon's 'Lucy in the Sky with Diamonds' and McCartney's 'Sgt. Pepper's Lonely Hearts Club Band' both have to be considered progressive pieces. The most progressive piece is 'A Day in the Life' which features both on vocals. So you have to give credit to both.
Most Likely To Be A Mainstream Rocker
It is tempting to choose Paul because he had the most mainstream post-Beatles career. Of course, George was very successful and widely considered a first-rate rock guitarist. But for my money, if rock and roll is supposed to be fun and carefree, then Ringo wins. He still occasionally throws together a rolling party called "The All-Starr Band" and takes it on tour.
First Hint of Prog Brilliance
There were hints hidden in the b-sides of 'Revolver' and 'Rubber Soul' before anyone ever really started to identify anything called Prog. But the real emergence of the Beatles' progressive side had to be:
Sergeant Pepper's Lonely Hearts Club Band was a departure from the majority of the Beatles' most popular work. It was also a concept album, in a way, and it was an attempt to push the creative boundaries of their work driven largely by a perceived competition with Brian Wilson of the Beach Boys whose complex vocal arrangements defied anything being done in music at that time.
Probably the infamous white album. It has some of the most technically complex music they made and at the same time, some of the silliest. One of rock's earliest two-record releases, it is still a subject of controversy given how little it resembles most of their other work. At the same time, there are moments on the album that have to be considered masterpieces, such as the Harrison icon, While My Guitar Gently Weeps.
Influenced By Them
It is safe to say, practically everyone who came after them was influenced by them. Their commercial success in the early sixties was unparalleled. Their prolific hit production was the envy of every other artist in that era. Their creativity and uniqueness in their later stages was amazing. Everyone wanted to be the Beatles. Everyone tried. No one succeeded. But their influence was enormous and to this day, new artists are affected by their songwriting and harmonies. Even more than that, however, was their influence over contemporaries. There were numerous successful bands in the 1960s that competed with the Beatles by trying to sound as much like them as possible. Their shadow over their present and the future was enormous.
No one will ever really refer to the Beatles as a Prog band no matter how progressive they were in their later albums. They were the lords of pop music in their time and as such, in terms of success, talent, and influence, they were unmatched in music history. The band was rightfully inducted into the Rock and Roll Hall of Fame's inaugural class. Since then, John Lennon, Paul McCartney, and George Harrison have also been inducted as solo artists.
My Personal Take
Like the Beach Boys, the Beatles were a staple of my earliest exposure to music by sharing a room with my older brother. He owned most of the early Beatles hits and I can truthfully say, I saw them in their early Ed Sullivan appearance on TV. I fondly remember their Saturday cartoon version, as well. So I can say that I was somewhat of a Beatles fan. That said, I must admit I never owned any of their albums, other than the White Album, until well into the CD era of adulthood. Nonetheless, I was very aware of the Beatles progressive work, or as my mother referred to it, "the drug years when they got weird." No matter how much you love songs like "Love Me Do" and "Eight Days a Week" there is something strangely compelling about "I am the Walrus" and beyond a doubt, "A Day in the Life" is easy to recognize as genius. I can vividly remember Paul McCartney's solo hit "Uncle Albert/Admiral Halsey" in the early 70s as a clear indication that the Beatles as solo artists were continuing with the 'weird' phase. It seems odd in retrospect that Paul, arguably the most TV-friendly Beatle, turned out to be the most prog-oriented of the bunch, but he clearly was. Lennon wrote much simpler songs in his solo days and 'Double Fantasy' brilliant as it is, is still remarkably simplistic in many ways. George wrote some fantastic pop hooks as a solo artist and his lyrics were often very contemplative, but to call him a progressive composer is a stretch. As for Ringo, I feel he's never gotten his due as a performer and as a drummer. But no, he's not prog.