For some 40 years Bell has explored the Convertiplane concept in which two or more engines and their associated propeller/rotor assemblies can be swiveled between the vertical and the horizontal to  provide direct lift or level thrust. Proved as early as 1958 by the XV-3 research machine, the concept was shown to be practical by the XV-15 of the late 1970's. Bell then teamed with Boeing for the JVX  (Joint Services Advanced Vertical Lift Aircraft), a multi role convertiplane based on XV-15 Technology. The first of six V-22 prototypes flew in helicopter mode during March 1989, and the first full conversion to plane mode was achieved in September. This type has very considerable capabilities, but the whole program has been troubled by changing political and economic commitments.  As of early 2000 three test aircraft had crashed: no one was killed in the 1991 crash, an accident in 1992 killed seven men, and the third in April 2000 killed 19 Marines.